1. Always test drive the car before buying it - with the seller! That way, you can put it through its paces and if the seller doesn't like it, then the deal's off.
2. Until recently, this wouldn't have occurred to me. Always check the mileage and make sure it's in kilometres (if, like me, you're buying one in Canada). Ask me how I know.
3. When checking out the car (oh yeah, always look at it first) bring a rubber mallet and make sure the body's stable. People are really good at making crappy cars look nice.
I'll be adding to this list as I go. Excuse me, though, I have to work on the car I just bought ....