I feel like the kind of people who post pleasant fantasies like this have never worked retail because oh my god no, please no do not do this. Not only are you slowing down the line and forcing someone who's trapped there to interact socially beyond the scope of their job, there's a 100% chance I lie to you about my favorite candy and I'm gonna throw away anything you give to me (if I'm even allowed to accept it, which nowhere I worked retail would have ever allowed that kind of liability) in case you've wiped something on it.

Creeps have ruined this kind of spontaneity, and until we fix society in general it's not coming back. It's a nice thought but please let it stay a thought.