And while you're at it, don't be a Yankee about it. Ask nicely if they mind taking the lock off. And if they don't have the tools you aren't actually in a gun shop or a pawn shop. You're in a florists and they are trying to sell you a dozen roses.
I will say that literally anytime a customer tries to start disassembling our guns themselves, I take it from them. A year or less of selling guns will prove the rightness of my actions to anyone. I've seen "gun guys" immediately dump a mag straight onto the ground, and a guy who has owned guns his whole life leave a very expensive pistol damaged at the muzzle from trying to remove a tightly fitted barrel bushing. So, from one gun seller to one potential buyer, don't be a ponce about it, and know what to look for without getting forensic.