I took the opposite sort of attitude whenever I end up selling an amplifier. Now and again I get stung when the owner of a repair job fails to pick it up and pay me. My recourse is to sell the thing and satisfy my "mechanic's lien" against the deadbeat's property.
The seller's risk is that the new owner will find a way to abuse it, blame the seller when it blows up and demand some sort of compensation, or return privilege.
If I sell the thing, I want it sold. I want it out of my sight at least for a reasonable service interval before it breaks again. I'm not running a used car lot ("Well if you don't like it try this one for a week").
My policy is for the buyer to bring his radio and his wattmeter to the shop before he can take it out of my building. This way I can find out for myself if the buyer's radio is too big. And whatever his wattmeter shows, he gets to see that for himself. Never mind what my wattmeter reads. His meter is bound to be at least a little different.
But those are the two major hazards to the seller. Excess drive wattage, and a guy who whistles, blows, and tweaks the knobs until he melts a tube, trying to get "just a little more" power out of the thing.
Let the seller beware.
73