When a car is showing as ‘being purchased’, it means that a customer has added it to their cart and started checkout. If this happens, the car will be unavailable until a purchase is made. If the customer changes their mind, the car will become available to buy again.
If you were in the middle of purchasing a Cazoo car and accidentally locked yourself out of checkout, just give it between 30 minutes and an hour and the car will become available again for you to purchase.