When buying a used air compressor, first download the manual and find out how long it takes to recharge the tank from zero. Then look online for valve rebuild kits and don't buy a compressor that does not have them commonly available.
Drain the tank, then time how long the recharge takes. If it is within a minute or two of the manual, you are probably in good shape.
If you take the head off and run it for 5 minutes (I know what you are thinking, it won't hurt anything), you should not see a puddle of oil forming above the pistons. If you do, it needs new rings.
If it takes forever to recharge, and the rings are good, you probably need new valves. You can order a valve kit (for good compressors, probably not the Husky) or take it to any local compressor shop.
The shop won't want the whole thing, just bring them the head. They will put new valves in it and send you home with a new gasket to use when you put the head back on.
For home use, a real air compressor, rebuilt, with yearly oil changes and a belt every so often, will probably last 15 years before another rebuild is necessary.