Buying a used air compressor?

I'm using a used one today. I've built them from scratch, bought them new, and have refurbished them for others.

Used is OK, probably the best bet.

Few minor things to look at;
Power requirements. Better be sure you have the right kind and amount of power.

If you go 110, you can pretty much ignore the inflated horsepower. Look at the electricity it draws, that will tell you the maximum true horsepower.
If it draws 15 amps at 110, it's about 2 real horsepower. I've seen them marked up to 7... but they still make about two.

If they are 220, much better, but again, check the power consumption.
Example I have a 3.2 horsepower Husky. My buddy bought a 7 horsepower Husky the same week. Both were bought used and I wired both of them the same day. He was a little disappointed that mine draws 100 watts more than his... makes more air volume as well.
Sold under different advertising rules.

So check your amps/watts for a realistic estimate of power.

Any brand name 60 gallon unit will treat you right if it is in decent shape.
Open the drain at the bottom. A little red in the water is no big thing. A gallon of rusty water, the tank has not been maintained.
Look for obvious damage, like the tank having been tipped over. Why take a chance? Look for another one.

Look at the oil. Has it been kept changed? Or is it new enough not to be dirty inside the sight glass?
Blow some air out onto something and give it a sniff. If it reeks of oil, you may want to keep looking. A very faint oil smell is not a big thing.

Many units are sold when the D-switch goes bad. D-switches are easy to change and quite reasonably priced.
Many units are sold when they leak. I don't mean holes in the tank, just leaking brass parts.
Those are very easy to replace and also quite reasonably priced.
The one way check valve that goes into the tank from the compressor is often bad. They are cheap.

Avoid any visible damage, avoid any dirty looking motors. Probably brushes going bad, makes for a dirty look.
Oil covered compressors should be skipped as well.

If you can't hear it run, it's worth half as much as if you can, at most.
See lots of them that 'run great' but you can't test it.

You can buy a new HF one pretty cheap, but DON'T buy a used HF unless it is dirt cheap. Try to get name brand, but some of the worst units I have seen were name brand, but thrashed.