How about a forum dedicated to this? It's probably been brought up before, but I didn't see it.

When people list their sites for sale they are usually ripped apart by the members. This way if someone wants to sell they can get all their (****)stuff together and have a price range in mind when they list.

Plus I think it would be fun to read and even if you don't list your site or domain, give you a decent idea of what it's worth.