I had a leaking oil pressure sender on my Nissan truck so since I could always hear the engine sound change at just before a quart low I was going to plug it off.
I got the "what's it for?" question and I told him it was just a brass metric pipe plug. (common as can be)
I made the mistake of not just walking out and told him what it was for and he refused to sell it to me, saying "I can't sell you that because doing what you want to do with it will endanger the engine".
I asked him if when he took the job did he have to take some sort of an oath of "Do No Harm" like a doctor does or what, and he got all shitty about it.
I left and got one at Ace Hardware.