The Collaborative International Dictionary of English v.0.48:

Bidder \Bid"der\, n. [AS. biddere. ]
   One who bids or offers a price. --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bidder
    n 1: someone who makes an offer
    2: someone who makes a bid at cards

Bouvier's Law Dictionary, Revised 6th Ed (1856):

BIDDER, contracts. One who makes an offer to pay a certain price for an
article which is for sale.
     2. The term is applied more particularly to a person who offers a price
for goods or other property, while up for sale at an auction. The bidder is
required to act in good faith, and any combination between him and others,
to prevent a fair competition, would avoid the sale made to himself.
     3. But there is nothing illegal in two or more persons agreeing
together to purchase a property at sheriff's sale, fixing a certain price
which they are. willing to give, and appointing one of their number to be
the bidder. 6 Watts & Serg. 122.
     4. Till the bid is accepted, the bidder may retract it. Vide articles,
Auction and Bid; 3 John. Cas. 29 6 John. R. 194; 8 John. R. 444 1 Fonbl. Eq.
b. 1, c. 4, Sec. 4, note (x).