I lost count of the number of eBay shocks I bought that were "perfect", "recently serviced", and "VGC", but in fact turned out to have all the damping attributes of a guitar string. Its a royal pain getting a decent used shock.
Air seals are a doddle to replace yourself, but damper servicing requires slightly more specialise equipment.
I have a 165mm shock that looks, and is, very well used, but it's got a damper that works and it still holds air. Its perfect for a winter hack, or for someone converting to air from steel spring to see if its right for him. You can spend £300 on a half decent fox air shock, but if it turns out that air is not for you (it behaves in a totally different way to a steel spring) then it's an expensive operation. My shock is available for the cost of postage, so its a very cheap way to find out and avoid an expensive mistake.
It's detailed in this thread;
viewtopic.php?f=46&t=285335