I have one of those. Bought it in 2005. I have replaced the belt once since then. I added a solberg silencer type filter to quiet it down. It has been a good machine. When run for a long time, the unloader valve blows a tiny bit of oily, rust colored "snot", but it has never needed an oil top up.
The last time I replaced it, the belt was available from sears parts.
It is a 3.1 HP compressor. around 11 SCFM@ 90 psi.
The OEM on that model is DeVilbiss.
The price he's asking would be fair if it was running. I'd talk him down to $200. You are not only looking at replacing the belt, but also taking the chance the belt broke due to a problem with the pump. You will not be able to test run it, so you are taking the gamble on it if you buy it.
If you buy it, and all it takes to get running is a belt, Give it a fresh oil change. Then adjust the pressure switch to shut down around 135 or so. From the factory they come set to 150psi. That is pretty high for a single stage pump, and unnecessarily hard on it. Beyond 135 psi, the pump output falls off rapidly, and it will run a lot longer from 135 to 150 than it did to get to 135 from 120.