Selling domain names that are already in use

On 27 February, I purchased a domain name and paid for it. On 5 March, after the domain still says pending, I decide to call. I give them my details and they advise me that the domain name is already assigned to another user but it is currently not active, so I ask how they can allow me to pay for it if it’s unassigned and they cannot give me a reason. They then call the other user who advises them that she wants to reactivate it. So my first question is, how can they sell me a domain name, and debit my card for it if it is assigned to another user? My second question is, if I didn’t contact them, would my domain be pending indefinitely? At the moment, I would definitely not recommend them.