So far, (knock on wood) I've been very lucky in my dealings in web auctions - In fact a time or two I felt I came out ahead. Example: I have carried a partially completed Airfix PBY around for several moves, always intending to finish it even though it had a bunch of lost/broken parts. So when that kit appeared in a web auction advertised as "believed to be largely complete but for parts only" I bid on it. 6 bucks including shipping - when it arrived and I opened it, I found two complete, unstarted kits in the box.
Nowadays, I don't do web auctions (I think its been a couple of years since I made a bid) cause there are too many people trying to make a living at it, too many people sniping, and entirely too many people trying to get money for nothing. And too many people willing to pay ridiculous prices for certain items (which generaly puts the item out of my reach). So now I haunt the flea markets (Airfix Lanc, complete in sealed box - $5), Yard sales, local consignment auctions (largely for tools, but I've picked up the occasional kit or model rr items) and of course the Hobby Lobby 50% off sales