It looks like you noted that all the product descriptions have been rewritten to be unique, so I don't really see a huge issue with this. I guess the better question to ask you is if the amazon/ebay pages you control are ranking, and if so are you getting sales out of them? I know everyone wants their native website to rank, but if you're selling products then the real KPI you need to be worried about is sales. If the amazon and ebay pages are getting you sales, then I would make it part of the strategy to keep those ranking above competitors.
"As effectively why would you show our site in the search results for a product range, when all of the content/products already appear on eBay/Amazon which is several SERPs places higher?"
That's a great question... what can your site provide users they may not be able to find on Amazon/eBay? Do you have more information about the product, buying guides, use cases, great stories about how other people have used the product (since I really don't know what you're selling), or other useful content? My take is that you need to provide a good reason for someone to visit your site, so don't fight Amazon if you're using it to sell. Ranking well doesn't keep your business in the black, sales do. Utilize Amazon and eBay as part of the marketing/sales strategy to push the competition out of the SERPs (1st page), and use the website to provide a unique experience which caters to your brand.
"Is Google not inclined to think, "hey no need to show x site, as the content is already features on Amazon, why show it twice?"
I would think that too if the content is the same or if the experience is the same. The question really is WHY should I visit your site? What can you offer me on your native site that I can't find from visiting your Amazon page? Amazon is a very trustworthy site, so If I'm going to buy something it might make sense for me to purchase there (especially if I can use Prime).
Think about it and let us know