I love ebay and have scored no end of amazing bags, clothes and rare vintage pieces I wouldn't find anywhere else. Since moving out of a big city, there are fewer physical outlets in which to track these pieces down, so I make the most of online resources when I can.
You really do have to do your homework. I follow feedback trails on all sides, along with sellers' ID histories and forms of payment accepted. I learn the authenticity signposts for each item, pay attention to the detail, care and even literacy involved in the text of each auction, and will not bid if the seller won't post photos of the actual item. I keep finding stock photos from other web sites in certain auctions and refuse to bid on them.
I also always check the "Completed" listings to see what similar items have actually sold for, and to see just how many times a certain item has been re-listed, and at what price. Doing this is actually more instructive than cruising the active listings, and paints a more complete picture. It also helps me get familiar with each seller's physical setting for photos, which enables me to know when they've stolen from another seller. If you click on the seller's other items, you should see the same background - it's always a red flag when it suddenly changes to something completely different.
If a seller doesn't reply to my queries, I pass them by. If they sell low-ticket, mundane items as a rule and then suddenly have a high-end bag for sale, I get suspicious and move on. I've been really lucky and have honed my radar to the point where I can usually tell from the gallery photo and auction title if it's the real deal. I've reported dozens of fake Hermes sellers and really try to stay alert and awake when looking for the good stuff.
There are a lot of honest sellers on ebay who really do give a damn about their products and service, as it's their sole income and they value their reputation. It's too bad that the thieves out there tend to cast them all in the same light.