Lisa offered to sell Mary a bookshelf at a garage sale for $200. Mary could not accept the offer at that time, but she did give Lisa $50 in return for her promise to keep her offer open for three weeks. Mary returned with the balance two weeks later to find that Lisa already sold the bookshelf she had offered to sell to her. Lisa explained that she had been able to get a better price from another buyer. She offered to return Mary's $50 and insisted that this was all she was obligated to do. Is Lisa right?