Yes, you should be looking although I don't know what price would be right for that model or make of mower. If it looks OK and its all there, ask for a trial and see what size of yard the the seller has. How much work has the mower been given? Does he look after his property? What's the oil like? Best if it is shows signs of use but not VERY dark. New oil would heighten suspicion that its never been changed before. Does it smoke (ie burn oil leaking from the valve stems and/ or bore? ) Is there a good reason for sale?
If they are looked after and kept dry between each use (and particularly in winter) mowers can last well into their second decade. Best of luck, Chris

Thanks Chris

just found out its a 1998. 15 years old now. I just dont know. They guy is asking $550.