1)(with apologies to above power sellers)No matter how many stars an ebayer has you are still subject to possible scams or false advertising of product condition.
2) Even if a product on ebay is offered with free shipping ebay returns are usually harder.
3)An ebayer could give you a negative rating for returning something while Amazon will usually take it back no questions asked.
4)Amazon shipping is usually faster and free or little cost
While you can definitely get some bargains on Ebay if prices are comparable(and many times they are) Amazon wins by a long shot.