In a horoscope, the fourth house is called Vahana Sthana (Vehicle position). Venus is the planet of Pleasure, Luxury, so naturally Venus becomes the karaka (significator) of the Vehicle. Mars represents the structural aspect of the Vehicle as Mars would represent metal, engineering, machine, etc. The function of the car or a vehicle is to transport people, or means of transport. Transportation is the use of the car. Mercury is the planet associated with transportation, short-distance travel, communication, etc. therefore, Mercury must also be considered when buying or selling a car.
The beneficial aspect in the fourth house, the fourth lord or Venus would be very useful for the purchase of a car when the main or favorable sub-class is working, or even for a good planetary transit over the fourth house. The association of the beneficent planet could be by aspect or conjunction.
Make sure these planets are in direct motion for as long as you buy a car. That is the fourth lord, Venus and Mercury. If those planets are affected, the structural and functional appearance of the car would be seriously affected in the future. The Ascendant when buying the car is also very important to take into account. It should also be free from afflictions. Who benefits from a sale, be it the seller or the buyer, etc., can also be seen in the positioning of these planets involved at the time of purchase.
For a good result, make sure that Venus, Mercury and your fourth lord in the horoscope are well placed.