Unfortunately desperate sellers will resort to all kinds of stupid things, like using stuff like StopLeak and fuel additives or whatever else they can come up with to temporily fix a problem.

If the person tells you something like, sorry but it hasn't been started in a while, and it runs and starts good, then there is a good chance that he is honest and it is a good running car that just has a few quirks now and then.

See if it smokes or stalls in all the different gears, and see if there is any uneven wear on the tires.