Kilns last decades, so the used market is genuine and worth using. What matters is knowing which faults are routine maintenance and which are the end of a machine, because the price of a used kiln tells you almost nothing about which of the two you are looking at.
What is routine and what is terminal
Elements, thermocouples, controllers and brick patching are all replaceable and all normal. A cracked or distorted shell, a collapsed lid or a chamber that has been fired well beyond its rating are not economically repairable. Ask what the last firing reached and how long it took: a kiln that took much longer than its rated schedule is telling you about tired elements rather than about a fault.
Parts availability decides value
A used kiln from a maker who still supplies elements, thermocouples and bricks for that model is worth substantially more than an equivalent chamber from a maker who does not, because the first is maintainable and the second is disposable. This is the single most useful question to ask a seller and it is often not in the advert.
The costs that come after the price
Transport, because kilns are heavy and awkward; an electrician if it needs a circuit; shelves and props if they are not included, which is a significant cost in itself; and probably a set of elements at some point. A cheap kiln with no furniture and tired elements can end up costing more than a modest new one.
Where they are sold
Suppliers list new kilns with specifications and lead times, and many also handle used machines and part exchanges. Studios closing down, colleges replacing equipment and potters retiring are the other route. Buying from a supplier costs more and comes with support and parts, which for a first kiln is usually worth the difference.
Questions people ask about kiln for sale
What should I check on a used kiln?
Elements for sagging or breaks, brick for structural damage, the lid or door seal, and what the last firing reached and how long it took.
Are parts available for old kilns?
For some makers yes and for others no, and it makes a large difference to what a used kiln is worth. Ask before buying.
What else costs money after the purchase?
Transport, possibly an electrician, shelves and props, and eventually elements. Furniture in particular is a significant separate cost.