Flippa and Digitalpoint. Flippa has the better site structure and payment process (in case you need a refund/review) as it's built for selling sites.
Digitalpoint is a forum so you will have to sort out payment externally, usually via paypal. You get some cowboys on there, nevertheless it's used by alot of digital marketing agencies (my last job) - which says alot : )
Last edited by Noreen; 08-13-2011 at 04:20 PM..