I prefer
phpAuction (but
not the GPL version available free online or in Fantastico!), or
phpProBid.
Some differences between them, but the important thing to me is the ability to charge variable fees for defined Services (type of listing, length of listing subscription, reserve prices, etc.), and payment gateways and admin modules.
COMPARISONS, REVIEWS, SPECS SIDE-BY-SIDE OF MOST SCRIPTS