As a reminder, you have to have a Google AdSense account to get an eBay account to make any money off of placing eBay ad capsules on your Hubs. If you don't have an AdSense account, you can only make money on HubPages with Amazon capsules.
EBay capsules would be useful if you are writing about a specific topic that would have similar items available on eBay.
For example, an article on personal finance may do better with an Amazon capsule with books on the topic you are writing about than an eBay capsule (but may work if you were writing about safes for in your home). An article on camping may do well with an eBay capsule for camping equipment (an Amazon capsule may work well here also).
Since I haven't been approved for an Adsense account yet, I tend to use mostly Amazon capsules, but I have used a few eBay capsules because I thought they fit the Hub. One down side of Amazon capsules, you don't make money just because your reader clicks on the ad. The reader has to actually purchase the item for you to make any money. Many hubbers feel that there is a low yield for Amazon capsules. I guess it depends on the specific topics you write about.