I guess like most things it depends on how well it was taken care of. My Taylor is about 8 years old and looks (and works) great. Granted, it's under a pole barn with sides near the stove; The anode rod has been changed out regularly, I clean it out too often, replace gaskets ahead of time, etc.
I was at the State Fair yesterday and the regional Taylor distributor had about six different stoves out for display (most were sold) and besides some soot coloring, and normal wear and tear, mine looks in pretty good condition. I will say there is a new (to me) design I may try to incorporate. I will also say that some (some) of the welding doesn't seem to be as nice as my older one- I'm sure they're fine and will wear well, but it's something that I noticed. It may be nothing or my lack of experience with welding.
I would suggest as another poster mentioned, seeing what comes with it.. pumps, PEX tubing, fittings, clean out brushes, anode rod (do you need to buy a new one now?), extra chemical treatment for the water, etc. Look at all the gaskets (there's one on the back) to see how they look, does the temp gauge work or not, if possible, drain and take the anode rod out and look inside the water jacket with a bright flashlight. Does the aquastat work? Did he have the stove on a relay? Are the pumps original? Is the blower fan original (and does it work)?
Did he heat his water with the stove (that little feature is great and can be added on, but at a cost). If the feature isn't' there, that's a good negotiating point- look online for the price of the coil (then add in a the tubing, maybe a pump, the work). You'll need this feature to heat your pool.
Taylor's are a pretty straight forward design (good and bad).
Good luck-
Skot