I've used both apptopia and flippa to sell apps within the past year or so. I sold one app on apptopia for $500, the sale came relatively quickly, maybe within a week or so of posting it on the site. However, I stopped using apptopia when they decided to start charging for listings, which were $50 or so last time I checked. I would only ever use them again if I knew my app was going to fetch a certain price and not just $500 like the one I sold did.
I also sold a bunch of apps in a group on flippa a couple of weeks ago. The interesting thing about this was that I no longer had the source code for the apps (so all I could offer was transferral of ownership in the app store), but even still, what I thought would only sell for $150 or so ended up going for $550.
I'd say both are great websites to use.
