I used Sedo in the past, some of the domains up there, you can make offers on them, and you wont get a reply. Sucks.
Professional wise, and the number of domains up there.. its pretty good. Some domain owners I think also have an unrealistic expectation on how much their domain is also worth.
I troll through the godaddy auctions, there are some nice ones in there from time to time, and cheap too.