If you're buying used, either get one from a reputable dealer, or if you're Ebaying CHECK THE FEEDBACK.
I bought my 394 from "saw-king" on Ebay, and it was exactly as described, to the "T".
If you're looking in "want ads", look at the saw for the usual signs of abuse: metal discoloration on the bar form over-heating. The chain should be sharp, whether he stuck a new one on or sharpened the one that was on it. Check out the air filter: does it look like it's never been serviced? Ask if they have reciepts for any maintainance that's been done. Ideally, bring a compression tester; if he's being honest, he won't mind. Barring that, remove the plug and see if the cylinder is junkola or not; it won't be easy, so bring a penlight- if he'll let you pull the muffler, that's even better. Does it start easily and run smooth?
I'm sure I'm missing something, but that's all I can think of right now.