If I bought a car and j6 months later found out that it was in an accident before I bought it, can I void the contract and get my money back?

The issue depends on whether the seller knew of the accident and hid that fact from you. Since a car's value is diminished, and it often has some ongoing problems,after an accident, the fact that a car was in an accident is often a material, or important, fact, which a seller would normally be considered to be under a duty to disclose. The failure to disclose such a fact could be fraud; and fraud may provide a basis to rescind the contract (return car, get money back).

However, fraud depends on a knowing misrepresentation or concealment--if the seller himself did not know, there is no liability. Also, not all accidents are considered equal--a 5 mph fender-bender in a parking lot which did no frame or structural damage might not, for example, be considered a material fact, whereas any accident which caused structural, motor, drivetrain, etc. damage clearly would be material.

So, there are situations under which you may be able to rescind this transaction. If you believe the accident was severe, and also the amount of money at stake is a few thousand dollars (e.g. this wasn't a "junker" bought for $800, which would not justify the cost of legal action), you should consult with an attorney.