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As Takeshi said, there really is no benefit of doing that with respects to SEO. But some people are desperate, ill-informed and willing to try anything.

Instead of immediately assuming Penguin there are things like Relevancy, localization, personalization, link authority, language tags, and so on that have affected results for some time. It could just be that the content, while in English and just as readily available to US users, has better bounce rate, better traffic, more authoritative links and so forth for Canadians and Australian search results. It could be that sites which are highly targeted toward US users have better/more relevant content that is pushing your sites down in the US but don't have the same reach in Canada & Australia. It could be that the terms you're losing traction on in the US serps are more volatile or have a different breadth of competition than in Canada or Australia.

First, determine if those duplicate content pages need to exist or if your users would be better served by another page. If that page doesn't need to exist then you may want to consider a 301 redirect to the better page. If a page is an exact replica of another page then you need to ask yourself "Why do we have it?" If its only a duplicate because of thin content then you might want to consider adding more, relevant content to the individual pages to better differentiate them.

If the duplicate page needs to stay for whatever reason then you can consider adding a canonical tag pointing to the primary page. Some cases in which canonicals have worked best on the sites I work on have been relating to parameters. E.G. example.com/product and example.com/product?model=4 are basically the same page but they each serve a purpose. In this case, example/com/product?model=4 is a subset of the one without a parameter and was given a canonical tag pointing to the primary page.

Canonical tags are a signal, not a directive though... which means that the search engines may choose to listen to it or ignore it as they see fit.

I apologize if any of that seems confusing. Here's a link to the SeoMoz guide on canonicals: http://www.seomoz.org/learn-seo/canonicalization and a blog post on the subject: http://www.seomoz.org/blog/canonical-url-tag-the-most-important-advancement-in-seo-practices-since-sitemaps

Ah, Wordpress. The Replytocom parameter is an annoyance but can easily be fixed. Its not so much that removing Disqus is causing Replytocom... Disqus was just masking its presence. Replytocom exists as part of the native WordPress commenting system. Go to Webmaster Tools > Crawl > URL Parameters. Choose to Edit Replytocom, choose "Yes: Changes, reorders, or narrows page content", under How does it affect page content you can choose Other, and then choose to have Googlbot crawl No URLs with this parameter. You can also add replytocom to the Robots.txt for the blog to disallow it. And If you're using Yoast there's an option under SEO > Permalinks to remove ?replytocom variables.

This sounds like an ambiguous area. Your FAQ is your FAQ and should stand on its own. Now, technically speaking, if the article is what you want ranking for your FAQ you could consider a canonical tag telling Google that your FAQ is a subset of the Article's superset.

You could also, instead of including the 300 word FAQ copy/pasted into the article, link to the FAQ from the article where relevant and/or only include a sentence or two from the FAQ that is pertinent to the article.

Yes, pointing from a subset to a superset of information is one of the main reasons for the canonical. So Google would not view it as improper. Just remember... Canonicals are a suggestion NOT a directive. Google will choose whether to listen to you or not.

It is usually better to redirect one to one instead of redirecting from a redirect. Where possible it should be avoided because eventually, if you chain too many redirects to redirects, Google will stop caring. There will be no negative impact from changing an old redirect to point at a more relevant page. You may want to double-check that the level of traffic and inbound links pointing to the page is worth going through the trouble of the 301 instead of letting it 404. Sometimes it can be better to just let a page die if no one is clicking through on it.

Much like Matthew, I feel that keeping the Archive links would depend on how else you're interlinking content for users and your personal preference. Odds are that your posts are tagged... so your users can find the older, related content that way. If you want a visual representation of how often you post to your site there's the Calendar widget and other similar plugins that will link to your older posts. You can have a date archive list of posts (but the longer you're around and posting, the more overwhelming that will get and add far too many links) or you can have a dropdown menu pointing to your date archives. Then, of course, there's a Search Bar... let users find what they want that way instead of offering up 4000 different ways to get to those archives. If you think your users will have a need for any of those and it adds to the user experience, then go right ahead with them. If they just clutter up you page and offer up little extra value, then there's no real need for them.

For SEO purposes the archives have little to no value, create duplicate content, and having all those links will just dilute link equity being passed. But its more important to consider its impact on ease of use for visitors. Ask yourself the following: Will this help visitors? Do we need 6 ways to get to the same thing? Is there a better way to show them the same information? Does it make my site more easily navigated or just clutter things up?

Either way you cut it, sounds like a poor man's linking scheme to me.

Have they considered, perhaps, one or two sites with a bunch of relevant categories? Somehow I feel like starting and running 100-200 legitimate shops like that would be more of a headache than just taking all the related products and slapping them together.

Since numerous search results pages are already in the index then Yes, you want to use the NoIndex tag instead of a disallow. The NoIndex tag will slowly lead to the pages being removed from the SERPs and the cache.

1. If you have older content and you create newer relevant content that you want people to see instead of the older content, you likely want a 301 redirect. In this way, all (mostly all) of the link equity is passed to the newer content which will eventually rank in place of the older content.

2. If you have duplicate pages like those caused by a parameter where site.com/page1 is the same as site.com/page1?this=x then you should canonicalize the page and its parameters to site.com/page1. In this way, the search engines understand that page1 is the real version of the content and thanks to the canonical will eventually take the place of the parametered versions that had been appearing in the SERPs.

Appendix to 1... down the road, those older pages that were redirect may wind up with no more links pointing to them from anywhere and no traffic going to them. At this point you may consider just 404ing the older page if you'd like to clean up older, less useful redirects.

Appendix to 2... A Canonical is a suggestion, not a directive. This means that the search engines do not have to follow it if they feel it is not entirely relevant.

Rel Canonical is not quite the right thing for this sort of issue.

If you're worried about the 404s sitting around too long and losing traffic for the moment, you can 302 everything to a landing page, category page, or homepage while you work on setting everything else up. You have two choices at this point.... 1) recreate all of the old pages and old URLs then remove the 302s, or 2) Add new products and new URLs, then as Dana said you'll need to map out all your new product URLs and old URLs to determine what old URL should be 301 redirected where. Then set up your necessary 301s and test that they all work.

Okay, so no 301s.... well then, assuming all of those duplicate sites have been live for a while and crawled by the spiders, you might be better served with canonical tags.

Have you checked the HTTP header status code shown to users and are you sure that its not just a custom 404 page? Could you give a specific URL as an example?

If the page doesn't exist and only offers a small amount of info like that then making it a 200 across the site when Googlebot sees it would cause Google to view it likely as duplicate thin content or a Soft 404. So a real 404, if it is in fact a 404, is the correct thing to do.

Personally I don't think the NoIndex, NoFollow solution is the best because they'll be harming or killing any link equity that was going to the site through that page instead of consolidating it. The best solution would be to come up with the means of adding that 301 as soon as you can so they stop hurting themselves needlessly.

Could they possibly leave the page up and just add some form of "Product Discontinued" type of message on the page to direct people to the most pertinent category page and other listing/product as needed? That way they don't shoot themselves in the foot while they wait on the ability to easily 301 and then once they can they just need to 301 the pages with the "Discontinued" message to one of the relevant pages listed.

Ah... its a search results page. Generally speaking, best practices for internal search results pages is to disallow them in robots.txt as Google usually considers is disfavorable to have search results appear in search results. What I'd really worry about here is that it could accidentally be viewed as cloaking since you're serving Google something completely different than you're serving human visitors. (Though a manual reviewer should see that you aren't doing it with malicious intent)

Does this only happen on search results pages?

Assuming they're trendy, I agree on the Social Media front. Trendy fashion items should have a good likelihood to be shared via facebook, twitter, instagram, pinterest and tumblr.

As for the generic terms to attempt ranking for, I'd say use them in a blog campaign to at least create some content devoted to those terms. Get enough people following and liking via social could lead them to linking your blog content in various places which could eventually boost rankings from having a strong, natural backlink profile. And who knows, something could go viral.

It may depend on how the tabs are set up. If you can see it in the page source without any problem then usually Google can too. Quick test to check: Grab a chunk of content, copy/paste it into Google search with quotes around it and see if your page comes up. If it did... then yes, Google read it perfectly fine. If not then you need to check how your tabs are hiding the content and fix it.

Two of the ecommerce sites I work on handle content on product pages using tabs to separate specifications, description, accessories and so on. Google can see all of our stuff perfectly fine as one page.

I'd rather deal with the duplicate content. Personally I'd bounce quicker with Thin or no content than I would with the same content on a different but similar product page. Of course I wouldn't let the duplicate content sit there and hurt me... I'd add canonicals to pages that were similar. Now if it was the exact same content everywhere then that'd drive me nuts. But if I can look at all the products, realize how many are the same with a minor variation and how many truly different product types... then I could write content for fewer pages and consolidate link equity with the canonical without worrying about duplicate content penalizing me. Of course I could always just NoIndex those duplicate pages instead.

4. Custom 404 page?