I really must be missing something with the OP. I used to have a small gun shop, you know, buy, sell, trade, repair. Bought hundreds of firearms or all types. Sold them, too.
But I never had a customer want to "step out back and let me fire it a few times". Nor did I try to shoot those firearms that I bought - would have kept me pretty busy!
People all the time eat a meal without taste-testing it first. Unless they are royalty, and have an official "taste tester/poisonee".
Maybe that is why prisons have gotten away from the electric chair - no one wanted to be the one that "test fired" it!
Cleaning a ML gun isn't all that difficult or time consuming - water, maybe some soap, and some clean patches, then a wee bit of oil. No disassembling required.