What's the law regarding someone selling something that belongs to someone else, without their knowledge?

My mother’s ex is selling his house, along with a static caravan that she paid for and which is in her name (it is included in both the house listing and photographs). She didn’t know he was doing so until she happened to see the listing in an estate agent’s window. Before she moved out he said he would pay her for the caravan as he wished to rent it out, but she has received no money for it. He now is asking her to pay to remove the caravan, and wants to bill her for both this removal and “fair rent for looking after it for best part of year” at £50 per week.

Is he allowed to have included the caravan in the listing?
And is he allowed to demand what he is for it?