She has appropriated it already via the act of selling it, she treats it as her own property, and also by selling it shows intent to deprive you of it. Bad darts from the cops to be honest, they will just have wanted to get the incident closed down. Best way to deal with it is to phone again, report a theft and also complain to the Inspector of the shift that attended about the lack of action from their officers. This is just based on what you have told us.