Welcome to the Q&A Forum

Ya, a 15% non-indexed rate is not bad. There was a Q&A here earlier that was looking at similar things: http://moz.com/community/q/some-urls-in-the-sitemap-not-indexed. That should help ease your mind! DA isn't a function of how many pages your site has, but how many other credible sites link to your site: http://moz.com/learn/seo/domain-authority.  Cheers!

The amount of code (navigation, related item links, reviews, etc) that eBay adds to its pages should prevent this as well as being selective about how much you send.  For example, you could write a longer description as part of content on your page while uploading a truncated one to eBay. Mostly it's a a question of sales though. If your store numbers remain healthy while eBay adds to it, then that's the most important measurement.

But, in the first place, why did you choose to NOINDEX, FOLLOW those product pages? If you have a preferred product page among those different variants seen as duplicate why don't you just use canonical to point there? What do you think is the benefit of noindexing? Theoretically leaving them there with the duplicate content and a canonical you are wasting some google bot crawling budget, but unless you need google to crawl your pages with a high frequency (because your content is frequently updated) I wouldn't care much. Personally I see de-indexing as the last resort.

Check my answer to similar question: http://moz.com/community/q/pages-with-rel-next-prev-still-crawling-as-duplicate Even if you add a canonical url, those pages are technically duplicated, that's why moz report will keep reporting them as duplicate, in the report the canonical url is shown in a different column, so if you don't care about them you can just filter them out.

Are you using Canonical tags on any of these pages? If you aren't, you should, it will help the duplicate content issue somewhat. If 1 product has 2 different URLs then it is technically 2 pages to the search engine. Canonical tags will help get rid of those errors as well. If the content is copy and pasted it is duplicate content. If the content is thin then the html coding that is probably very similar on both pages is what the bots consider duplicate content because it is the only content. Your text to html ratio should be much greater. The engines don't need copy to be 100% identical in order to classify it as duplicate content. Those errors are important because it is telling you that the engines don't see those pages as relevant because they aren't uniquely valuable to the searchers.

At a certain point, it becomes extremely difficult to justify expending time on creating quality unique descriptions for individual products unless you can get enough unique reviews specific to each product page. Because of this, you need to make a decision as to whether you think you can keep them indexed.  If you do an outstanding job on every other aspect of your site's SEO, you may be able to, however it's not a sure thing even if you do all that other work. What may be more beneficial is to noindex the individual flame sticker product pages and focus more energy on building trust / ranking signals focused on their category page.  There is no "one answer fits all" scenario unfortunately. As for your Domain Authority, you'll need to speak with a Moz rep for more clarity, however Domain Authority is based on Moz's proprietary formula for understanding overall quality of the full spectrum of SEO signals.  So the better you do across the board in each aspect of SEO, the stronger that score will be. You can learn more about DA on their explanation page.

I'm afraid its a job to be done manually (you can bulk upload the keywords so don't panic too much!) you can find out how to here - http://moz.com/help/guides/search-overview/keyword-rankings/add-and-manage-keywords 1,000 is rather a lot is may be worth looking if you need that many e.g. are they all getting month searches etc.  depending on what Moz plan you've got you may not have enough keywords in your campaign be able to add that many. Take a look at http://moz.com/beginners-guide-to-seo/keyword-research if you want some ideas Hope that helps.