Top One:

http://www.auctionmac.com/ The obvious place to go, very user friendly auction, easy navigation; different categories. With auctions @ 0,50 US$

Links selection:

http://www.macbid.com/ the Mac only auction, nice and cool site 

http://www.auctiondepot.com Mac and pc's, busy place, simple but very efficient site http://www.ubid.com/ general auction site, with some impressive Macs @ impressive prices