A car is typically considered used once the buyer completes the paperwork and drives the car off the lot. Once it is driven by an owner other than the dealer, it has a vehicle history unknown to the dealer.
Even if someone just purchased a vehicle, it is immediately considered a used vehicle. When someone is looking to purchase a vehicle, there are a number of variables to consider like accident history, wear and tear usage and possible mechanical issues. These could potentially be issues even if the car has only been owned for a short time. However, if the vehicle is still under warranty, this improves its stock. One possible exception to a car becoming used instantly after purchase is that some dealers allow a three-day cooling-off period where the buyer has the option to return the vehicle. However, dealers are not legally required to provide this option.