Buying used can always be a crap shoot - make sure you can return it!
Guitars especially. I got a good deal on an Epi
Dot Studio on Ebay ($130), only needed a set up, had a few small dings in the body, other than that it's great and equal to the ones I see used at GC for $200.
See my Taylor thread for an ebay sale nightmare, luckily ebay 'buyer protection' can work!
Bought a used Fender CD140SCE electro/acoustic last year at GC, and paid the $20 for extended warranty. (Side note: I had tried out dozens of e/a's for months in the $300-$400 price range, so knew what I wanted.) When the pickup sounded bad, brought it in to the GC tech (Nashua guy is pretty good), he popped a new battery in, could not find anything wrong with it. In the meantime I had already called the ext warranty place, and because it was under $200, they just refunded the whole deal, including the warranty cost! So now I've got a good guitar AND a GC gift card to use up!
Bought a used Kawai synth and a behringer GFX 2x12 amp used from GC last year, also put the ext warranties on them, sold the amp for my cost, and the synth just sold on ebay for more than my cost (although the person has not paid or responded to my message about pickup yet).