Have a 3-way conference call where you explain straight out to all parties involved how the deal will play out. You can give the title(s) to the transporter, and he is not to offload your property until he sees the vehicle you are to get and the title that goes with it. And I’m sure you and the purchaser will be required to pay some upfront money to insure the transport payment, whether your transaction goes through or not. His time and mileage are not free if you two don’t complete the deal. To be honest, it’s ridiculous way to handle a deal, way too much can go wrong trading out assets sight unseen.