You're always taking a risk when you buy used... Me, I've bought about 6 Macs from eBay and only one had serious issues. (A white iBook G3, which are known to be the most unreliable Mac out there; unfortunately, I was ignorant of that fact when I bought it.) Other then that, no problems at all. I'd recommend it if you do your research on what you're buying first.
Oh, and FYI: don't buy a computer because of the software. It's best to buy one that's been wiped clean, or buy one and do it yourself.