First the easy question. He can’t sell the caravan because it isn’t his. To do so would amount to theft. As for charging rent, I don’t think he has much of a case, since he never informed your mother about the rental price beforehand. From a legal point of view, though not a moral one, you may have trouble talking about the fact that he promised to buy it, since he will probably deny it and it then becomes a mater of his word against your mother’s.