I buy and fix used gear, and I walk in with three checks that take less time than the drive over.
One: power it up and listen at low volume. Crackling that vanishes when you exercise the controls is dirt, the cheap kind of problem. Hum that sits there no matter where the volume knob is points at power or ground, the pricier kind. One silent channel can be either; find out before you negotiate.
Two: look inside if the seller allows it. You are not judging tidy versus messy. You are looking for leaking batteries, corrosion, burnt smells, and evidence that someone has already been inside with a soldering iron and good intentions. Previous repairs are not disqualifying, but they belong in the price conversation.
Three: completeness and the real model number. Knobs, cabinet, feet, manual. Missing pieces become a scavenger hunt later. Photograph the label on the back; the name on the front is often a series, not a model, and the label is what parts catalogs actually use.
None of this makes a piece valuable. It makes the price honest, for both sides.
What is your own pre-purchase ritual? And what is the best, or worst, surprise a check ever caught before money changed hands?