This is nothing new, but works pretty good for me.
Use Ebay Pulse to identify good selling products and create keywords from them.
If you identify a product niche you like, do a search for completed auctions. That will tell you if there are a lot of completed sales and how much they are selling for.
Then do a normal eBay search and the search result page will list related terms below the search box.
You can apply the same niche search on Amazon.
Search for the hottest selling products on Amazon and then use the product names to create long tail keywords.
You can then take those products and search for them on Ebay to see how they are selling and get related search terms.
Code:
http://www.amazon.com/gp/bestsellers
Once you have an idea for a target keyword, go to the goog keyword tool and do a search and get an idea of how many searches per month a term is getting plus related keywords.
Code:
https://adwords.google.com/select/KeywordToolExternal
You can apply this technique to a BANS site, Amazon affiliate or Adsense.
The hard part is determining how heavy your competition is for a particular keyword. And I have not run across any free software that will analyze how well a keyword is SEO'd. I have only recently started using Micro Niche Finder and it does that analysis. So I can decide if it is worth the effort to target a particular long tail keyword.