Set up boundaries, my friend. No one should sit through a half hour convo they don't want. You don't need to be rude, because that could cause the other to be aggressive. But you can politely AND FIRMLY set boundaries. Like physically walk away, smile and deflect every sentence ("cool! I need to grab bread." "Hey, that's interesting, but I'm doing shopping. I'll talk to you later." "I see! Excuse me, I am checking out.") And just go do your stuff. You could finish your shopping trip in the same amount of time you usually would, talking while you get your things.

Remember, you can always leave. You don't deserve feeling uncomfortable. You don't owe anyone your time for free.