You shouldn't back out of an agreement.
Tony finds a car he likes for sell in the paper and agrees to purchase it from the seller when he gets paid that Friday.
Tony wants to get a reliable car at an affordable price.
Tony gets paid Friday and is able to get the seller to take off another $200 dollars when he pays for the car.
Tony buys a cheaper car the next day and never tells the previous seller he doesn't want his car.
The seller uses the funds from the sell of his car to catch up on some overdue bills.
The seller sees Tony at a store in the other vehicle and keys his car because he's angry at him for his behavior.