Is it permissible to sell the vehicle and take the down payment after the buyer's disappearance and cessation of communication, and is his absence considered a repudiation of the sale, knowing that his whereabouts or phone number are unknown, according to the Maliki school of thought?

The Maliki school of thought dictates that earnest money (urbun) sales are invalid, and the seller must return the earnest money to the buyer. This is also the view of Al-Shafi'i and the jurists (Ahl al-Ra'y). If the agreement stipulates taking the earnest money if the sale is not completed, then this contract is voided in the Maliki school. Your sale of the cart is valid due to the invalidity of the first sale, and the earnest money must be returned to its owner. If it is not returned and you despair of its return, then you should give it as charity on his behalf.