I don't do eBay here in HubPages. But in Squidoo, I have found it to be useful. Whenever you are promoting any products from Amazon, check if the same product is available at eBay (most probably, there will be).
So let's say a visitor comes and clicks to both Amazon and eBay. He may get intrigued by eBay product more because of various reasons like: free shipping, more warranty or even because of his desire to pay by PayPal (Amazon does not have PayPal option).
So, that's the point. In HubPages, your revenue won't increase by getting clicks. But in Squidoo, the lensrank increases with click-outs, which can ultimately mean the difference of over $40 for a single page.