yes it is basically to keep their identity hidden. You can see who won at the end of the auction. On ebay uk, you dont even see a bit if the username once the auction passes 100 pounds, you just see bidder1, bidder2 etc
That sucks. I asked a question about adding a BIN to an auction and all of a sudden, within a couple hours of the question (and right before the seller responded), there was a bid on it. I looked on the bidder's history and 5% of their bids were for the same seller. Could be legit. Could not. It just looks suspicious IMO.