I use ebay all the time to buy and sell. I have to agree - there are millions of transactions a day that go well. Out of the ones I have participated in buying something, only one was not as advertised and, because it was a very expensive item, I pushed the guy hard for a refund which he sent. Out of the hundreds of items I have sold, I only had one problem and it was beyond my control and the guy got a full refund and still left me negative feedback.
However, that being said, I have heard that many, many fake purses are sold on ebay as real. As a previous poster said, if you must have the Coach, find it on sale locally so you know it is real. If you feel you really need to buy it on ebay, check the seller's feedback and also carefully read their return policies, if they have one.