How much does an accident devalue car? accident was 3 years ago, car is great. Don't know how much less I should be asking for it.

There's no specific dollar amount per accident, the type of damage and extent of will make a difference. Having proof of what was replaced, and by whom helps, vs "take my word for it". Someone slid on ice and you needed a new bumper cover? Few Hundred maybe, took out the front end of the car, couple thousand. I would look at a number between average NADA, KBB, Edmunds, whichever you prefer lists for suggested trade and suggested private party prices and be ready to get lower offers. Its the overall condition of the car, appearance and mechanically that concern a buyer, most cars have a reported accident, some major, some minor. If there was no frame damage and it was fixed proper I wouldn't worry if the owner had done proper maintenance and cared for the vehicle otherwise.