As stated, it's an OK deal IF it is in good condition.
Here's how to test it (assuming that seller can demonstrate): have tank completely empty and start up the compressor - listen for any knocking sounds from the pump - if the rods have bearing slop, you will hear it right at startup up until there is 30+ psi in the tank. Once the pistons are pushing against air pressure, the rods are held against the crank and the noise quiets down. Also check the pump oil level and color - should look like clean motor oil. Check tank for moisture and any oil (shouldn't be any oil in the tank and hopefully not much moisture). If you get a mocha-like mixture out of the tank drain or if that's what the pump oil looks like, I'd run away.
You can also do the time test for fillup (honestly don't know how long it should take for this unit but it's not that hard to calculate).
If you do end up getting it, I'd go into the pressure switch and dial back the pressure cutoff to 120psi (or maybe even 110-115). Single-stage pumps really struggle to get much over 110psi and it will be much easier on the unit if it doesn't have to run up to 135 (motor and pump will both last longer). Quincy on their single-stage pumps only rates them at 100psi maximum, and those are much heavier-duty pumps than this one.
