Just stumbled on this thread during a search. Here's a bit more info for the 602:
I'd have no qualms about picking up a used 602, but it helps to know what one is looking for. Typically someone is selling the stove because it doesn't quite work the way it used to. That is often due to age or deferred maintenance. This usually shows up in warped and or cracked burn plates. These parts are sacrificial and last about 10-15 years with normal burning. The good news is that they are replaceable without a rebuild. With new burn plates, the stove becomes just about like new. If you can get an older 602 for under $250 that is in good shape except for needing burn plate replacement, and it fits your needs, I'd go for it.
Another thing to look for is a crack in the back, under the vent pipe. Usually this happens if it is overfired or if it has been exposed to too much heat. If someone has tried to cram too much wood into the firebox it's possible to displace the top burn plate. If this happens at the back of the stove then the firepath is all at the back of the stove and it can overheat there. Other than that normal wear and tear are what one should watch for. They have great enameling, so if the enamel has several flaws, chips or cracks, I'd consider that a sign of neglect.
The good news is that this stove is really simple. I can tell its condition in a 1-2 minute inspection. Once you are familiar with them, they are very easy to run and maintain. If stove is in primo condition and shows little use, I would pay up to about $500 for one. If you find a stove you are considering and can take some shots of the stove, the interior and the back, I'd be happy to review them for you.