I just recently was looking for a Coach purse and had the same dilema. I ended up purchasing mine locally at a department store, b/c I knew how upset I would be if I bought a fake.
I was very glad I ended up going that route; when I went to the Coach store in our mall and asked about the fabric I was interested in ,I found it doesn't even come in some of the styles that were listed on eBay. :O Namely, the style I really liked! She let me look throught the book and said I could special order anything they had, and it wasn't there. I asked if it had come in that style and she told me no.
I guess my point woud be, go in with a lot of knowledge. Go to a local retailer and look at the purse you want and get a good idea of what it looks like, etc - then compare. And of course check feedback!!
Good luck!