So, a man and his wife go to the beach everyday and everyday there see a woman with a bag going around to people as they sit under their umbrellas listening to their radios and try to sell them something. Since most people react negatively toward her, the man and the woman assume she's selling drugs. They decide to find out what she's selling and one day the husband brings his radio and umbrella to the beach since she normally doesn't approach them when they just come and sit under the sun and play in the water. She approaches him and then leaves moments later. The wife approached her husband and asks what she's selling. The husband smiles. She sells batteries for radios.
He says, "She sells C cells by the sea shore."