Bing and Yahoo are converting much better for our ecommerce site..... even though we're PAYING Google.
People may be searching, and not finding and going to other search engines.
Today I did a search for a particular product with the manufacturer name and part number. The first site was amazon, followed by about 15 various amazon affiliate BS pages from facebook, blogspot and others, two pages in I found two REAL sites that had the product listed, followed by about 15 more BS affiliate pages. This was a US brand and a very popular product. Now I'm not talking about Google shopping, in fact no shopping results showed up, nor was there any indicator as to "Hey, you might find this on Google Shopping... here's a link" ... nothing but BS posts.
Amazon has to be paying Google for this type of search results... there's really no other explanation at all.
I wouldn't list on Amazon ever, they'd have your sales stats and start selling those items themselves, they've done it before and will continue to do so. Ebay is an option and we do sell our returned open box items there quite well.