The second-hand wheel market in the UK is supplied by potters retiring, colleges upgrading and people who bought a wheel and did not continue. The last group is the best source: machines with almost no hours on them, sold because a hobby did not take.
Where the good ones come from
Barely used domestic wheels, sold within a year or two of purchase, are common and are frequently as good as new. College and studio machines have had far more use but were usually maintained, and often come with bats and tools. A wheel that has sat in a damp garage unused for a decade is the one to be careful of, because water rather than use is what ruins them.
What to pay for
A supported maker, smooth bearings, a working pedal and included bats. What not to pay for: cosmetic condition, which has no bearing on throwing, and an unusual brand with no parts, however clean it looks. A scruffy wheel from a known maker beats a pristine unknown one.
Testing it before you buy
Ask to run it. Centre something if the seller will allow it, or at least press firmly on the head while it turns to feel whether it slows. Rock the head for bearing play, and work the pedal slowly. Five minutes tells you nearly everything, and a seller unwilling to let you is answering the question anyway.
After you get it home
Clean it properly, check and tighten fixings, and replace the belt if it looks perished. A wheel that has been standing benefits from being run for a while before you rely on it. Set it at the right height for you before the first session rather than after.
Questions people ask about pottery wheel
Where do used pottery wheels come from?
Mostly people who stopped after a year or two, and from colleges and studios upgrading. The first group often sells nearly unused machines.
What should I avoid?
Wheels that have stood in damp storage, and unusual brands with no parts availability however good they look.
How do I test one?
Run it, feel for bearing play by rocking the head, and work the pedal from stopped. Five minutes is enough.