Is buying and selling cars to a customer, then reselling them to another merchant—where the customer pays an agreed-upon increase called "car depreciation"—considered a legitimate trade or usury?

If the car is sold in a true sale, entering into the ownership and responsibility of the buyer, then there is no harm in selling it for more than its price. It is permissible for the buyer to authorize you to sell it after purchasing it, so that he can pay you its price from the loan he takes from the bank. However, if the matter is merely granting a loan with an increase, then this is forbidden usury (riba). The decisive factor is the reality of the transaction; if it is a true sale, there is no harm, otherwise, there is. Assisting in usury is forbidden, and even if the transaction between you is permissible, it is not permissible to assist someone who intends to use it to obtain a usurious loan.