My experience has been that it's usually not worth buying it back unless the damage is easy to fix and if you think that the car is worth more then they're offering you find some realistically priced ads for same year comparable car to show what it would cost you to replace it and they'll usually work with you.

EDIT: Like not_a_charger said the ads had to be from somewhat within your area, similar mileage, only a year off at most, similar condition etc..