I'm a sucker for jokes like this

A couple lived near the ocean and used to walk the beach a lot. One summer they 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, 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. The couple assumed she was selling drugs and debated calling the cops, but since they didn't know for sure they just continued to watch her. After a couple of weeks the wife asked, "Honey, have you ever noticed that she only goes up to people with boom boxes and other electronic devices?" He hadn't and said so. Then she said, "Tomorrow I want you to get a towel and our big radio and go lie out on the beach. Then we can find out what she's really doing." Well, the plan went off without a hitch, and the wife was almost hopping up and down with anticipation when she saw the girl talk to her husband and then leave. The man walked up the beach and met his wife at the road. "Well, is she selling drugs?" she asked excitedly." No, she's not." he said, enjoying this probably more than he should have. "Well, what is it, then?" his wife fairly shrieked. The man grinned and said. "Her name is Sally and she's a battery salesperson." "Batteries?" cried the wife. "Yes," he replied. "She sells C cells by the Seashore