I think I would use Projects vs. Classes...but think either should work.
Purchase Car:
DR Inventory (Project A)
CR Bank/Cash
Maintenance/Repair:
DR Maint/Repair (in COGS) - Project A
CR Bank/Cash
Sell Car to Customer:
DR Bank/Cash
CR Revenue - Project A
Rustler recommends using Items, not sure what the advantage is, maybe automates this part of entry.
DR COGS - Project A
CR Inventory (Project A)
However, by doing above, the best you will be able to see is Gross Profit by car. In order to see actual net profit you would need to be allocating Indirect cost (like Office rent, phones, salaries, etc.) to each car.