a person employed to bid at an auction in order to raise the prices for the auctioneer or seller.
Origin of by-bidder
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a bidder at an auction who bids up the price of an item for the benefit of a seller
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012