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?