Beach Vendor's Secret

When we lived there, my wife and I used to walk the beach a lot. One summer, we noticed a girl who was at the beach pretty much every day. She wasn't unusual, nor was the travel bag she carried, except for one thing; she would approach people who were sitting on the beach, glance around furtively, then speak to them. Generally, the people would respond negatively, and she would wander off, but occasionally someone would nod, and there would be a quick exchange of money for something she carried in her bag. My wife and I assumed she was selling drugs and debated calling the cops, but we didn't know for sure, so we just continued to watch her. After a couple of weeks, my wife said, "Honey, have you ever noticed that she only goes up to people with boom boxes and other electronic devices?" I hadn't, and I said so. Then she said, "Tomorrow I want you to get a towel and our big radio and go lay out on the beach. Then we can find out what she's really doing." Well, the plan went off without a hitch, and my wife was almost hopping up and down with anticipation when she saw the girl talk to me and then leave. I walked up the beach and met her at the road. "Well? Is she selling drugs?" she asked, excitement pouring out with her voice. "No, she's not," I said, enjoying this probably more than I should have. "Well? What is it, then? What does she do?" my wife fairly shrieked. I smirked and said, "She sells 'C' cells by the sea shore."