Similar to mine. If she had said it with a smile on her face and a friendly tone, I would have thought she was just trying to be funny, so I would have laughed, nodded and confirmed it sarcastically and jokingly and just said "yes" as I walked away.
But if she'd said it with a more serious expression and a judgmental tone, I would have looked her in the eye and said in a sarcastic tone "uh-huh..." And walked away. The "uh-huh" would not be to confirm or deny her story, but it would have been a bit of an out loud thought for myself just saying "this woman is trying to put me down and she's rudely making observations about my private purchase. My purchases are helping the store. I am not stealing or doing inappropriate things in life. She's out of line. I'm not engaging with crazy. And uh-huh, yes, boy is she one of the rude crazies. Get me out of here before she keeps poking into what is not her business." The "uh-huh" is the confirmation to the bully that "yes, I heard and understood you. I am not slinking away in shame. You are not going to insult me. And you are not worth my time to banter with. I owe you no defense."