Kirby is right; the letters taken to use for the first and last that you see are randomly selected from the username, but they are ALWAYS the same letters when you see the user bidding on items. So if I saw someone bid with w****a on an auction, then if I see the same obfuscated username bidding on a similar type of item, then it's 99.9% certain it's the same person.

Take QoF for example. When I go to see what she's bid on, her bids are always listed with the same two letters at the start and end. So you can still track bidders.

Actually there's an easier way to find out who won. Just search by bidder with completed auctions ON and list if not winner ON against someone you knew who bid on the auction, and the full username of the winner is displayed unless the auction was private.