SP is sitepoint (sitepoint.com) If you're selling a cheaper domain, try DP for higher end ones, try SP (I've seen quite a few go for a lot there)
definitely site sales! Have you seen their sold sites off the premium site sale section? It's quite shocking! (the figures some of these sites go for)
I sell all of my things like that through DP, but ebay is also a good place to sell that kind of stuff. you could also go down the route of local classified ads too like freeads.co.uk or adtrader.co.uk
You can do both domain and site sales at sedo. Their rates are 10% for selling domains, not sure about websites though. A serious company to deal with, although rates may seem a bit high.
Depends what you're selling..but best way is to sell it on all the forums together, that way you have better chances ~MG
If you are just flipping template sites, I'd go with any of the sites listed above. If you have a true business then I'd consult some business brokers. They will do a valuation using tools like the Discounted Cash Flow approach. You don't end up with offers of less than a year's profit if you use proper valuation tools.
DP is already a good source of buyers but i have heard that you may can get a higher bid at ST. so why not post in both.