An online auction system for auctioning off products includes a server system, plural buyer systems, and plural seller systems. The server system comprises a seller interface, a buyer interface, a price adjustment mechanism, and a bid manager. The seller interface performs an online bid setup process including obtaining, from a given seller system, price varying information and quantity information corresponding to the price varying information. The buyer interface communicates, to select buyer systems, bid information and product information, and accepts from a given buyer system a given quantity needed and a given bid. The price adjustment mechanism determines a given parity price for the given buyer, which varies according to the given quantity needed. The bid manager defines a given winning bid for the given buyer as a function of the given parity price and a rank of the given buyer in relation to other buyers.