I'm not sure how you can physically tell if a bag is real or not. One thing you can do is look at the seller's ebay ratings. Many of the handbag buyers can tell if the bag is fake. If the person has a lot of negative ratings because they sold fake Louis Vuitton bag under the pretenses that it was real, don't buy for them!
I'm not sure how these sellers get these bags for so cheap. I've heard varying stories. One of the most popular is that a department store (let's just say Bloomingdale's) was unable to sell the bag for some reason. They will sell it to the ebay person for a lower price which is how you can get these less expensive bags. It could be that a department store ordered too much inventory and sold off their extras to the ebay person. Or, it could be that it was taken off the back of the truck (five-finger merchandise
).