Auction car prices are affected by many different factors, such as the age of the car. Other factors include custom work and documentation kept by the previous owner.
Auction car prices fall into two categories: classic auctions and simple used car auctions. Classic car auctions often bring the most money. These are the auctions where the car's value is largely determined by whether or not it is a collector car.
Factors that affect classic car auctions include restoration work and whether or not original parts were used in the restoration. Many classic car owners restore cars but do not use original parts, driving the price down.
Restoration work that is done using original parts brings more money because the car is kept in its original state, including the original paint job. If the paint is changed and not the same as what the car should be, the price goes down.
Modifications to classic cars work in either direction. In some cases, modifications increase the value of the car, particularly modifications that add speed. On the other hand, some modifications cause the price of an auction car to go far below what the cost of the modifications, causing the owner to lose money.