As of 2/13/08, The top Auction/Market Place Sites in terms of number of listings are:

eBay 12,833,061
iOffer 2,257,306
Bidville 1,074,027
eCrater 935,997
CQout 597,787
us.ebid.net 484,286
uk.ebid.net 466,247
bluejay 308,377
OnlineAuction 237,294
ePier 174,265
SpecialistAuctions 119,871
tazbar 83,310
wagglepop 77,957
AuctionQuests 66,905
PlunderHere 55,530
HiBidder 44,382

Go here for updates on this list:http://www.powersellersunite.com/auctionsitewatch.php