eggraid's advice is good -- just like a used car, take it in to a good shop and have 'em look it over. Checking out previous sales can be a good start, but if they bike has been upgraded with new / aftermarket parts, that can affect the price pretty significantly. The price will be determined both by overall condition and by the quality of components on it. Based on that info, your LBS should be able to give you an idea of what a reasonable price for it is. I'm sure they'd rather sell you something instead, but as long as you showed interest that you'd come back to them for servicing and purchases, I'm sure they'll be willing to help you out.
Also, make sure the bike is the right size for you before buying. Again, the LBS can assist if you are unsure.
Cheers, Chris