With the option to write a car off once the damage cost is too high, I think there should be an online scrapyard, where the cars written off by other players go, and you can browse the scrapyard to salvage another player's scrapped car. You can also scrap your own cars here, obviously. If you accidently scrap a car, you can go to the scrapyard and look at the list of cars you have scrapped, to see if they're still available or not. If they are, you have the option to salvage the car back, however you still have to pay the price that any other player would if they salvaged it. The cars would be set into categorys too, like in real life, where a car that isn't so badly damaged but was scrapped by a player who didn't have a lot of money, is a Category D car, easily repairable if you have the money. Then it goes all the way up to Category A, which is where you'll need a lot of money to save the Category A cars from the crusher. Also - if the game considers your car as a Category A write off, you are given the option to send it straight to the crusher.
So basically, I'm implying an ability to restore cars as well, but rather restore actual scrapped cars that have been written off by other players. Once you have restored the car, you can either keep it, or sell it on - although once you have bought a scrapped car, you can't sell it until you fully restore it - or, you could always send it back to the scrapyard.
Different cars would have different values in the scrapyard, for example, a more popular car wouldn't cost as much as a rarer classic car. Race cars would also be of higher value, depending on the damage.
Oh, and depending on the category of the car, it's available for only a certain amount of days.
Category D: 28 days
Category C: 21 days
Category B: 14 days
Category A: 7 days
Once the availability time of the car has expired, it's sent to the crusher. When you look at your list of cars, their status will either be 'Available', 'Salvaged' or 'Crushed'.
I'm probably planning way too much here for nothing, but once you get my imagination going, there's no stopping it.