Love this thread. I've been looking to find answer to this question. I used to get more quality results from Flippa when I wanted to buy sites, back when they were Sitepoint. Now it's more or less for newbies to drop their money.
For sales, Flippa offers a brokerage service as well. They don't charge, unless the site is sold. I've got an extension for a CMS that stably makes 6-figures profit for past 3 years. Last year I wanted to experience the process and test the market, but didn't find the right buyer. This year might try again. If interested, reach me on Twitter: @ugurkaner