Digital point was always a place when I came first when I wanted to trade sites and domains but lately it seems to be loosing pace to Flippa. Although there was some development in that area that part of the forum still looks like it could use a serious overhaul. The funniest thing is flippa adsense ads appearing all over the place right there in auction posts.
One of my ad in flippa ended with no taker ( wastage of $19), alternatively I sold it out through DP. I think DP is far better than Flippa because it won't charge for ad placement in specific section.
I haven't had any experience using Flippa, but I have been actively browsing Flippa. I find that a lot of auctions do not get a lot of bids. However, with the auctions that I have placed on DigitalPoint, I for the most part have had great success.