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It's always hard to talk in generalities about complex issues like this, but it sounds like a situation where cross-domain canonicals might make sense. I guess it really boils down to whether you're having issues with the duplicates and what the scope is (are there 3 of each or 300). In some cases, those duplicates just mean that one site will win, and Google will pick the winner. In other cases, the main site could actually be harmed by the duplicates. In some cases, honestly, multiple sites might rank fine. It really varies wildly. The cross-domain canonicals would help prevent any kind of duplicate penalty (like being hit by Panda), but it would also mean that the non-canonical versions would no longer rank. So, you'd be protecting the strongest site for each listing, but possibly cutting off the smaller sites. I haven't seen an implementation where different sites were canonical for different listings/articles/etc., at least not on large-scale, so that's a bit tougher to predict. If you have sites A-Z, and A is canonical for one listing, B for another, C for another, etc., that could get a bit tricky. I know large organizations, like newspapers, who syndicate content, have had good results in many cases with cross-domain canonical. There is also a syndication-source tag, but that's really a weaker tag, and I haven't seen much data on it. The other option, traditionally, would be a solid link-back strategy (the non-canonical versions link to the canonical version). Unfortunately, at large scale, that could start to make you look like a link network, so I think that gets risky in this case.

Does Schema.org markup require a HTML 5 doctype? No. These are signals/data for search engines, not coding that will be rendered by browsers.

Consider reversing your thinking from "what will be my loss to panda" into "what can I do to make this site kick ass". Reach for opportunity, extend yourself. If this was my site I would get a writer on those product descriptions to make them unquestionably unique, beef them up, add salesmanship and optimize them for search.  This will give you substantive unique content, that converts better, pulls more long tail traffic and moves out of competition with other sites that do the minimal. Sure, it will cost money but in the long run it could bring back a huge return. My only caution on this is that if you make this investment in writing you need to do that on a site that has can pull reasonable traffic.  If you do this on a site that has no links it will not do you much good.  It is part of a marketing plan not a single item on a "to do" list.

Open a ticket. http://seomoz.zendesk.com/home They are pretty good and should get back to you in a day or 2.

My suggestion: redirect mobile browsers to the HTML version as well as for the search bots. If the browser is not a search bot or a mobile device, and has Flash installed then redirect to the Flash version (not using a 301!) with a parameter to take the person to the same content on the Flash site (in the swf you can read the parameter and load the relevant part). Mobile users or search bots arriving via those links with parameters can be 301'd to the HTML version. Now mobile users and search bot get a nice HTML version. Flash users get the 'rich' experience. Search engines can crawl all the content nicely. Furthermore, it would stand up to a manual review. Many more details in answer to a similar question here: http://www.seomoz.org/q/converse-com-flash-and-html-version-of-site-bad-idea

We also just published a YouMoz on the main blog today that deals exactly with this subject. Check it out at http://www.seomoz.org/blog/3-steps-to-social-seo-for-your-boring-ass-clients

Just wanted to leave a quick note saying that SEOmoz has upgraded our Link Directory! You can view a post Cyrus wrote with more information about the directory update and link building via directories at http://www.seomoz.org/blog/seo-link-directory-best-practices