Any big bank or lawyer can function as an escrow agent with a specified technician to evaluate the piece on objective terms. Once the tech verifies, the funds would be released. Whether a seller would want to jump those hoops is another question even if you as buyer are paying the escrow fee and tech fee. I often had exotic cars subject to a pre-purchase inspection at my expense by a trusted mechanic--this is common in car world, probably not so much in audio.
Is there such thing as a test lab/escrow service for selling/buying audio gear?
I haven't had much luck selling used audio gear in the past. Sadly, it's not safe to invite a potential buyer into your home to test before buying, which excludes most of the popular local free listing services. I have tried eBay but the fees are high and buyers can claim shipping damage if they want to return purchases. (I had a very expensive amp dropped on the ground to claim shipping damage by a buyer who tried to modify the amp to power 400 cycle equipment.) I have actually tried to list gear on this site but gave up trying to get a listing posted. It would be nice if I could sell items and then send them to an independent lab where condition could be confirmed. They could be sent on to the buyer from there. It would even be better if there were an escrow service tied to the process which would release funds to the seller on the labs approval. Any thoughts or does anyone know of such a service?