Used strobes, speedlights and modifiers — fired a hundred times before they go out.
Flash tubes age and capacitors dry out, and neither is visible from the outside. We fire every used strobe at full power in sequence and watch the recycle time, then check colour consistency across the run.
Modifiers are checked for burn marks, rod damage and rot in the diffusion fabric, which is the part that usually goes first.
Bring the camera you will trigger it with. Compatibility between brands of trigger and body is the most common reason a cheap used kit turns out not to be one.