I have sold 2 websites on Flippa so far. Not impressed with the current pool of potential buyers, though. The majority are time wasters who only want to either spy on your site's keywords or haggle your price to a ridiculous low.
As for my first sale, I only listed it because I needed their Escrow feature. I met the buyer on Facebook, and I had to list on Flippa to make the deal secure for both parties. My second sale was earlier in the year. I listed the site because I was desperate to sell it. I was able to sell eventually, but not after I've had tons of ridiculous offers.
If you want to sell sites at fair prices, I'd recommend Motion Invest (losing its reputation fast, though). You can also meet quality buyers on Facebook Groups (I recommend "Flipping Websites"). But if your site is earning above $2,000 monthly, your best bet is Empire Flippers.