A: Anyone can always make an offer, and the seller could even accept their offer while yours is pending. However, if all the conditions of the contract have been fulfilled, or in a house/condo sale, are in the process of being fulfilled, then the seller would likely be liable to you for money damages if he sold it to someone else. Some courts may, if they find the contract valid and that the seller breached it, enforce the contract and force the seller to sell it to you - this remedy is much rarer though.
A: It was probably repossessed then. Call the tow truck company and find out why they towed it. If you owed money to someone for the truck, and you fell behind on payments, then they have the right to come and take the truck back. If you are current on all your payments, then find an attorney because either the tow truck company or the lien holder is liable for certain torts, i.e., conversion, wrongful repossession, etc.