I don't like the term 'flipping'. A website is more than just a bunch of bits, it is usually associated with a community of users and you don't just 'flip' the investment that 100's if not not 100's of thousands have made in you.

You need to take in to account the party that receives what you've created and you have to make sure that that party has your users best interests at heart. If not you'll end up with some money in your pocket but a tarnished reputation.

And if you buy websites purely speculative you have to ask yourself if there isn't a market better suited to that kind of behaviour.