What to do if I bought a used truck from a dealer and after the sale they informed me that it was in a accident?

If the car dealership advised you AFTER the sale that the vehicle that you just purchased was in an accident, then under the laws of all states in this country you have the righht to cancel (rescind) the purchase of the vehicle and get your money back.

The laws in this country is are that the seller of an item is required to disclose to all potential buyers all KNOWN matters pertaining to the item that may affect price paid or desirability before the sale, not afterwards.