The Collaborative International Dictionary
By-bidder
By-bidder \By"-bid`der\, n. One who bids at an auction in behalf of the auctioneer or owner, for the purpose of running up the price of articles.
Wiktionary
by-bidder
n. (context US English) One who bids at an auction on behalf of the auctioneer or owner, for the purpose of driving up the price.