I think as long as you know what to look for and do your homework before purchasing a bag on ebay, you should be ok... I know some ppl have had problems buying bags on ebay before, and ebay is definitely loaded with fakes. The problem is, it's really unfair to authentic sellers on ebay. Me and many other tpfers have sold on ebay and we do our best to show authenticity of the bags.
My advice:
1. Read vogue's authenticating page first, from top to bottom and memorize it. If you have time to kill, read thru the auth thread, there are several pointers in there.
2. Make sure the auction you are interested has all the necessary pictures, request them if you need to.
3. Check the seller's past selling history for duplicate auctions, that's a sign of a fake seller. If you can, search this forum for the seller's name and see if any results pop up. And don't forget to use goofbay to check for poor feedback.
4. Then, post any auction you are interested in in the Auth this thread.
Ebay is a valuable marketing community, it's just too bad that there are bad seeds out there that ruin it for everyone. Ebay has recently tried to crack down on fake sellers by limiting how many auctions they can list in a certain time period. The good thing is, if you know what to look for in authentic bags, you can always get a great deal.