Kilns wear out over time, each firing degrades the elements which will eventually need replacing.
The most important things to look for in a used kiln is the condition of the firebricks, and the elements. Usually there will be some ware and tear with a used kiln. However, major cracks in the lid or base, or elements hanging out of the bricks should raise alarm bells. Kilns wear out over time, each firing degrades the elements which will eventually need replacing.