Just use your common sense: if the price is too good to be true, it probably is. Research other similar bags to see what their prices are/were. You can look at closed deals to see what the average closing bids were. And always look at the pictures, and compare them to photos of the real thing. I saw a Louis Vuitton bag on ebay once that lacked several key characteristics of the actual bag, so it was easily spotted as a fake.

Also, never deal with anyone with a low rating, even if they are all good. (And check to see what else they've sold. There are instances where someone sells a bunch of low value items, and then puts an expensive Louis Vuitton up for sale. This is usually a scam. Or at least a warning sign. People bid thinking they are safe because of the good rating, but it was a set up.)

That said, I've bought several bags off ebay, and have yet to be disappointed.