by-bidder

by-bid·der

(bī′bĭd′ər)
n.
A person who bids at an auction to raise prices for the owner.

by′-bid′ding n.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

by-bidder

n
(Commerce) a bidder at an auction who bids up the price of an item for the benefit of a seller
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014