Every bid by every bidder on every listing is hidden in the same manner as all the other bids on that particular listing (a***b type alias, "Private listing--bidderS' identitIES protected" or a translation of the latter). You see your own ID in full only because YOU are the one looking; log out and navigate back to a refreshed Bid History Page and you'll see your ID alias. Only you and the seller (and I assume at least some eBay employees under at least some circumstances) can see your full ID. Same goes for Feedback Profile pages--where you see your ID on the seller's Feedback Profile everyone else sees an alias.
