I'm not on the clock.

I work at a retail store. I was on my lunch break and buying a few things so I didn't have to remember to buy them later. I was waiting in line, holding my items and a shopper stuck an item in my face and asked me a question about it. I said, "I'm not on the clock." He got offended and tried to insist that I answer his "one little question". I just repeated the same sentence. He tried to start insisting again when the shopper that was in line in front of me looked at him and said, "Damn, dude." He went about 6 feet over to someone who was on the clock and got his question answered. I am not trying to violate policy or deal with people's questions on my lunch.