Rather Obvious Shill Bidder

I bid on a small lot of medieval coins on eBay Germany and was
considering increasing my bid until I studied my competition: 2,561
bids in last 30 days, on 326 different items, 100% with this seller
only, and feedback of 14. I let him win, so presumably his feedback
will go all the way up to 15!

Usually it is not quite so obvious as this. The seller puts in a few
sure-loser bids in other people's auctions (below gold value, below
face value, etc., when that is possible), so you cannot actually
report them. But the alarms go off when I find a collector who bids
on Greek coins, Roman coins, Byzantine coins, Spanish coins, German
coins, etc., etc.--whatever the seller offers--but hardly ever wins.