by-bidder

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person who bids at an auction to raise prices for the owner.

from The Century Dictionary.

• noun A person employed at public auctions to bid on articles put up for sale, in order that the seller may obtain higher prices.

from the GNU version of the Collaborative International Dictionary of English.

• noun U.S. One who bids at an auction in behalf of the auctioneer or owner, for the purpose of running up the price of articles.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun US One who bids at an auction on behalf of the auctioneer or owner, for the purpose of driving up the price.