Welcome! Best advice is to look the machine over the way you would a tractor or other equipment. A couple of things you might try...
Of course, if possible, have the seller actually mill a log... or at least start up the engine. Run a straight line along the track. If it is bent or bowed, you may need straighten or replace it. Make sure the head moves up & down smoothly, and that the carriage rolls smoothly down the track. Bandwheels should turn freely when the clutch is disengaged. It looks like a basic manual mill, so if the price is right, I don't think you'll run into any issues that you can't take care of. As far as milling, set up a good foundation for it, and practice on a few low-value logs & cut some blocking & stickers for air drying the better logs. Try to keep the blade above the clamps & stops so you don't cut into them. Get a good cant hook, and you should be in business. Also, get at least a dozen extra blades, and either get a sharpener or find a good sharpening service (should be some locally, since you're in PA). Keep us posted & post some photos if you get it!