Macworld has posted a great list of tips for buying a used Mac:
Never buy a Mac that’s undervalued unless it’s coming from a family member or a longtime friend you trust. How do you know what the going rate for a used Mac is? Check eBay. Look under the “Completed Listings” link, which will appear in the lower-left area of the window after you’ve searched for something. Searching eBay and craigslist.org (which is in various cities around the country) is a great way to see what the acceptable price range for an item is.
When buying the Mac, consider whether you’ll need to put any additional money into it for repairs, possibly including an older Airport card (not the Airport Extreme, which some older Macs, including the iBook G3, do not support), additional RAM, or anything else. That, plus whatever you’ve budgeted to spend, should give you a more realistic idea of what your total expenditure will be.
In addition to their suggestions, you can always have an Apple-Authorized Service Provider take a look under the hood. The Genius Bar usually doesn’t do that sort of thing, but individual shops are usually more than happy to help a new Mac owner.