How do I determine depreciation value and my rights for a pre-ownedcar purchase?

To calculate the price evidence, you'll need to look for websites that provide information about the price (or price depreciation) of cars after an accident; you could also contact car dealers and ask to get written quotes for the trade in value of your car with the accident, and compare to the price you purchased it for.

If there was a written term in the contract that stated that the car had never been in an accident, you could allege that the dealer violated the contract by selling you a car that had been in one  and use that as grounds to try to get out of the contract--though you could expect a fight, since it's very doubtful that the dealership will be willing to give up the sale entirely. Without a written term about accidents, it would be more difficult to void the contract entirely, though if you at  least have the original CarFax, you'd be on good grounds to sue for a price reduction based on the value difference, if the dealership does not voluntarily negotiate some reduction.