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Yes, almost all of the link juice passes through with a 301 redirect.  A little is lost, but a very tiny amount.

Source: Matt Cutts http://www.mattcutts.com/blog/rel-canonical-html-head/

If you have your listings available in an unordered list, that should be fine.  If there aren't hundreds and hundreds of listings on your site, I don't think Google will have a problem with your implementation.  If there are, you might consider building static pages for each category, and linking to the listings from there.

Is there some alternate navigation to reach all of these listings without using your AJAX search?  Or are the listings included in a sitemap?  Is there some way for Google to find them already?

I'd recommend reading http://code.google.com/web/ajaxcrawling/ to learn more about how to make your AJAXy pages indexable.  You may also want to take a look at http://googlewebmastercentral.blogspot.com/2011/09/pagination-with-relnext-and-relprev.html if you have prev and next pagination.  If you have a view all, and want to make that the canonical form, you'll want to look at http://googlewebmastercentral.blogspot.com/2011/09/view-all-in-search-results.html

Also, in Bing Webmaster Tools, you can go to the Crawl > Crawl Settings tab and enable the "Configure your site to have bingbot crawl escaped fragmented URLs containing #!." option if that's applicable to you.

Google can read PDFs, and returns them in search results, but some users might prefer to view an HTML version.  Also, it looks like images in PDFs are not indexed, according to the 2nd post below.

Regarding duplicate content, Google says (2nd post below):

Q: Is it considered duplicate content if I have a copy of my pages in both HTML and PDF?
A: Whenever possible, we recommend serving a single copy of your content. If this isn’t possible, make sure you indicate your preferred version by, for example, including the preferred URL in your Sitemap or by specifying the canonical version in the HTML or in the HTTP headers of the PDF resource. For more tips, read our Help Center article about canonicalization.

These will be of interest to you:

http://www.google.com/support/forum/p/Webmasters/thread?tid=4472512a5515686b&hl=en&fid=4472512a5515686b00047d6de91c24fa&hltp=2

http://googlewebmastercentral.blogspot.com/2011/09/pdfs-in-google-search-results.html

As far as I know, you can't do this in Google Webmaster tools.  It would have to be a parameter like shopby= instead of shopby/ as you suspect.

This sounds like a good place to use the rel=canonical tag.  You want those pages to exist, and if people link to them, you want them to pass their link juice back to the /tshirts/ page.  These shopby pages will have a subset of the info on the /tshirts/ page, so Google should respect the canonical tag.

Other search engines beyond Google should respect the canonical tag as well, so that's another benefit to doing it outside of Google Webmaster Tools.

Here's some reading:

rel=canonical: http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html

Using canonical to a view all page: http://googlewebmastercentral.blogspot.com/2011/09/view-all-in-search-results.html

Google just announced some tags to help support pagination better.  They say if you have a view all option that doesn't take too long to load, searchers generally prefer that, so you can rel=canonical to that page.  However, if you don't have a view all page, then you can put these nifty rel="next" and rel="prev" tags in to let Google know your page has pagination, and where the next and previous pages are.

It sounds like you don't want to go the view all route, so you should read the second post below and you can implement the rel="next" and rel="prev" tags.

(from the view all post below) However, if you strongly desire your view-all page not to appear in search results: 1) make sure the component pages in the series don’t include rel=”canonical” to the view-all page, and 2) mark the view-all page as “noindex” using any of the standard methods.

View all: http://googlewebmastercentral.blo...

next/prev: http://googlewebmastercentral.blo...

Google just announced some tags to help support pagination better.  They say if you have a view all option that doesn't take too long to load, searchers generally prefer that, so you can rel=canonical to that page from your series pages.  However, if you don't have a view all page, then you can put these nifty rel="next" and rel="prev" tags in to let Google know your page has pagination, and where the next and previous pages are.

View all: http://googlewebmastercentral.blogspot.com/2011/09/view-all-in-search-results.html

next/prev: http://googlewebmastercentral.blogspot.com/2011/09/pagination-with-relnext-and-relprev.html

Google just announced some tags to help support pagination better.  They say if you have a view all option that doesn't take too long to load, searchers generally prefer that, so you can rel=canonical to that page.  However, if you don't have a view all page, then you can put these nifty rel="next" and rel="prev" tags in to let Google know your page has pagination, and where the next and previous pages are.

View all: http://googlewebmastercentral.blogspot.com/2011/09/view-all-in-search-results.html

next/prev: http://googlewebmastercentral.blogspot.com/2011/09/pagination-with-relnext-and-relprev.html

Oh, and I just came across this http://www.sabahan.com/2007/08/06/find-out-how-many-subscribers-a-blog-has-even-when-its-feedcount-is-hidden/... if they're using FeedBurner, and if the blog owner has the FeedCount activated in his FeedBurner account, you'll be able to view the actual number by following the instructions given there.  The article is a bit old so I'm not sure if it'll work; I didn't try it myself.

If they have an RSS feed, I think you can look it up in Google Reader.  Go to Google Reader, click "Explore" in the left nav, click Search, and enter criteria to find the blog.  It'll show you the subscription numbers for that blog.  If you have URL for the RSS feed, you can add that subscription to your reader account, click on it in the left nav, and then in the upper right hand corner, click the "Show details" link to see the number of subscribers.

According to this, this is only Google Reader subscribers, not all subscribers.  Google Reader is pretty popular, so this is probably a fairly good share of the entire pool of subscribers.

A bot?  You're scaring me a bit, like the lunch lady in Billy Madison.  Are you going to write a bot to spam your links on their comment pages?  That sounds fruitless... as these comment pages are already filled from top to bottom with spam.

If you use Firebug in Firefox, or Chrome, you can write what text you want in the field and add the link, then inspect the input where you see the text, and grab all the HTML that was generated by adding the text.  Then it's just a matter of having a bot inject that text into the page.  You'll still have to beat their captcha though to fully automate the process.

I tried it and it worked...

My guess is you're not putting the "http://" in the Link URL field, so the comment thinks it's a relative URL, so it ends up amending whatever you put after the page URL in the link text.  Make sure to put the absolute URL in the Link URL field, starting with "http://", and it should work fine.

You can do it the same way you can add links in for comments in this Q&A forum.  Write the anchor text in the comment, then select the text, click on the link button, and then put the anchor text in the Link URL field in the pop-up that appears.

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Hey Antonio!

It looks like all the elements on each page are in the source when the page loads... so I'm not sure where AJAX is being used.  There's a lot of JavaScript/CSS effects, but I don't see anything where JavaScript is loading something after the initial page load.

Usually the best thing to do is use subdirectories on one domain, to consolidate your domain authority/strength.  Here's some reading if you want to learn more:

• A recent Q&A about it with a staff endorsed answer: http://www.seomoz.org/q/subdomains-vs-subfolders-for-unique-categories-topics
• An SEOMoz blog post about it from 2009: http://www.seomoz.org/blog/understanding-root-domains-subdomains-vs-subfolders-microsites
• A presentation mostly pertaining to these choices for international sub-sites, but still relevant: http://www.distilled.net/blog/seo/mozcon-international-seo/

It can be done.  Make sure your development team reads through http://www.seomoz.org/blog/how-to-allow-google-to-crawl-ajax-content and http://code.google.com/web/ajaxcrawling/ before coming to any conclusions.  If you have an AJAX based website, you'll have a little extra work to do depending on the route you take.

Since you have no control over those other websites, you can't really do anything to get them to remove the links.  But because of this, incoming links to your site will never penalize your site.  If they did, you're right, a competitor could get sites penalized on purpose and point lots of links from them to your site.

What else could have caused your site to get penalized?  Could one of the Panda updates have caused it?  Check out the time line here and see if it lines up to when your site lost its traffic: http://www.seomoz.org/google-algorithm-change.

Hey JB!  Sorry, maybe I'm missing something, but if the pages are non-spidered and not included in search results, are you suggesting that Google is going to know about these pages and exit rates by using your Google Analytics data against you? Although Google is hush-hush about what they use Analytics data for, I don't think their intention is to use it for search results.  Here they say:

Shared data will be used to improve the services we provide you and will help create more powerful features for you to choose from. As they become available, only those who share their data with Google will gain access to these services and features (e.g. benchmarking and an enhanced version of AdWords Conversion Optimizer). The DoubleClick Ad Planner Publisher Center will also offer greater insight to the customers who have opted in to share their data in Analytics and Ad Planner.

Definitely 301 redirecting them is the way to go.  Make sure each page on the redirected domain maps to the corresponding page on your main domain.

It sounds like you know a few pages on these domains you want to redirect that rank well.  I'd pick one of these domains to start with, and redirect all of its pages to the corresponding pages on your main domain.  Wait until Google reindexes those pages that rank well, and try those searches again.  That page on your main domain should rank at least as well as the page on the old domain, and hopefully even better since their link juices are consolidated.

If you're highly risk-averse, you can continue doing this domain by domain.  Otherwise, you can batch them and do several at once, or just redirect the rest of them.  Just make a note of the searches where pages on these other domains were doing well, and check that those pages on your main domain does just as well after those pages get reindexed.  Personally, if it worked for one, I'd feel pretty safe doing all of them, and then keep checking the SERPs afterward to make sure nothing bad happened.

How gradual the process is will be determined by how quickly the pages are reindexed.  If they're reindexed quickly, you'll be able to see if it's working and can keep moving forward.

rel canonical is for letting search engines know that a page on your site has duplicate content of another page.  However, I doubt all the pages on your site are duplicates of your home page, so the most likely outcome is that the search engines would ignore your rel canonical tags and just index the pages as normal if they're not similar enough.

This is not really the intent of the tag, and to me it sounds like it falls close to the black-hat side of things.  If you really want the rankings from those pages, and don't care about them, you can 301 redirect them to your home page like Joshua suggested.

Here's the SEOMoz post from about a year ago about rel canonical.  It also links to some good resources: http://www.seomoz.org/blog/complete-guide-to-rel-canonical-how-to-and-why-not