Simple fix - Refuse the return. I make it clear, NO RETURNS, PERIOD. I've sold on Reverb nearly since its inception, and I've not once accepted a return. The only requests I got were so frivolous, (and we're talking maybe 2 or 3), that Reverb sided with me. I have over 350 transactions, preferred seller, quick responder, quick shipper, the whole 9 yards. It goes a long way. Most people who try to pull shit are typically not the brightest individuals. People think they can return a guitar because the action's too low? Too high? A JVM is noisy on OD2 Red? Yeah, no shit! (These are true examples). I now put a disclaimer about guitars set-up being strictly personal preference, and emphatically suggest a pro setup upon arrival.
Don't forget, when you sell on Reverb and a PayPal dispute ensues, REVERB becomes the seller, NOT you. And all the extra you pay for seller protection goes into a pot and gets used to pay off low-lifes to get them to go away with their silliness.
As a seller, I've had very few problems. Only real problems I've had were as a buyer. People list stuff as new or mint, and are clearly not when you get the item. From bad wiring, cracks, non-functional, buying new and getting used, that kind of nonsense.