Spare parts sales.

[Sarchez]

This could create a secondary income stream, as you service/sell replacement parts - for the vehicles you've sold years ago. (Current rating of an old car, could come into play with frequency you are selling parts.).

I know this opens up for setting prices for spare parts (or atleast an index figure), that should have an effect on sales of new cars. (High sparepart prices, might scare some people off and vice versa.)