I would advise anyone buying a mac to buy it at an Apple store if at all possible. Get them to order it in, and open it up and run it in shop. My dad bought a computer for his school, and was able (with a bit of pressure) to convince the guy at the Apple store to agree to replace the 17" Powerbook (About 2 years ago) with a new one, if there was even one dodgy pixel. Just a few days ago, I would say one of the contacts to the screen has stopped transmitting, or been damaged, and the screen now has a vertical red line down the screen. Luckily, the computer had the A.P.P. so it'll get a new screen, but that's off topic.
The point is, it's always a good idea to pull out the computer at the store and have a look. Makes it easier to get it replaced if there's anything wrong with it, because firstly you can "bargain" with the guy who's selling it to you, and there won't be any issues with whether you caused the problem or not. You won't get either of those benefits buying over the net from home.