A couple of caveats:
1. Most websites offered on Flippa are absolute junk. Seriously, if the sites offered there were so promising and offered such good potential earnings, don't you think people would be hanging on to them? Best website buys are usually through a direct approach to an asset you want that isn't even being offered for sale.
2. Website buys for SEO purposes are fraught with dangers. Firstly, it's well known that some sellers fake PageRank values using a known redirect flaw in Google, thus scamming buyers. Also, a change in WHOIS data can result in a domain's value being reset by Google anyway. This can especially happen where the domain ends up with different content or else is 301'd to another domain.
3. Also be aware that it's to be expected that some of the sellers will be linking to a website for sale from a personal network and/or link buys, that are all removed after sale, resulting in a domain with few if any real links to them.
I've bought and sold websites on Flippa before, and there are some gems, but they are far and few between.
Recently I've sold off a number of websites and domains, but I've done them through private sales to web design/development companies - better quality of transactions and better assurances of quality.
2c.
