There is no marketplace as big as Flippa for websites if you're talking quantity of sales. If you're talking high-end sales, very few happen in Flippa, they happen in bizquest, businessesforsale, sunbeltnetwork and other business listing sites.
Apart from Flippa, there's DP, and there are numerous other webmaster forums like V7N etc that are free. A simple Google search for "website selling forums" should lead you to them. But consider less conventional methods - try posting a page on your own site with a link from the homepage. Existing visitors may be interested ... and those who try Google for terms like "this website is for sale". There are also numerous website brokers who'll take your site on if it's a certain value.
