Most used studio kilns come from potters retiring, studios closing or colleges upgrading, and most are perfectly serviceable machines with wear in predictable places. Inspecting one is a short, specific job if you know what you are looking at.
What to look at in person
Open it. Elements should sit in their grooves without sagging out or showing breaks. Bricks chip at edges harmlessly, and cracks running through a wall or a distorted chamber are different. Look at the lid or door seal, the hinge and the base. Then look at the controller and ask to see it power up, even if the kiln cannot be fired where it stands.
The questions that matter more than the inspection
What did the last firing reach and how long did it take against the schedule it should follow. When were elements last replaced. Has the thermocouple been changed. Are parts still made for this model. Those four answers tell you more about the machine's remaining life than any amount of looking.
What is cheap to fix
Elements, thermocouples, a controller and brick patching. All are consumable parts on any kiln and all are available for machines from established makers. Factoring a set of elements into the price of an older kiln is prudent rather than pessimistic.
Getting it home
Kilns are heavy, top heavy and awkward, and a top loader will not go up a narrow staircase without several people. Arrange the move before agreeing the purchase, and check that the kiln can physically reach the place it will stand, including doorways and steps, which is a mistake made more often than it should be.
Questions people ask about kiln for sale
Is a used kiln a good idea?
Usually, from a maker whose parts are still available. Elements, thermocouples and controllers are all replaceable.
What faults are serious?
A cracked or distorted shell and a collapsed lid. Chipped brick edges and tired elements are routine.
How do I move one?
With several people and a plan, having checked doorways, steps and the final position first. They are heavy and top heavy.