Flippa is quite good but there are lots of people who try to sell low quality websites for a high price and some even purchase traffic to make their website look active. I have seen people sell websites that look active but actually it is fake traffic. You can easily see this by looking at their past sales. You see their old ads page that says their website has an alexa of 34,000 which was sold like 2 years ago and then now it has an alexa of about 5,000,000.
I like buying websites on active webmaster forums. Of course, I always look at reviews before buying and analyse the website to make sure it has potential. The disadvantage with this is that you don’t have a middleman so you can get scammed but chances are, you will use an escrow service for large transactions. A few examples of good webmaster forums are Digital Point, v7n.