Create a cost of goods sold account, say ‘vehicle resale costs’ to track any service costs
    Allow project allocations

Create an inventory asset account called ‘inventory – cars’
    Allow project allocations

Make sure the regular inventory asset account allows project allocations
    
Create a cost of goods sold account, say ‘inventory car costs’
    Allow project allocations

Use the VIN (or part thereof) as an identifier

For each car
    Create an ‘in progress’ project called Project {vin}

    Create an inventory item called Car {vin}
        Asset account = ‘inventory – cars’
        COGS account = ‘inventory car costs’
        Variance account = ‘inventory car costs’

When you purchase the vehicle, record the project on the purchase invoice

When you purchase stock items to repair the vehicle (batteries, mufflers, etc), record the project on the purchase invoice

When you purchase services for the vehicle (cleaning, advertising, etc)
    record the project on the purchase invoice
    the account will be ‘vehicle resale costs’ created above
    
You can get a list of current costs by running ‘Reports, project, allocation’ for car {vin}

When the vehicle is ready for sale, this report will itemize the vehicle assembly to create
    Create an inventory item called Vehicle {vin} – this is what we will sell
        Asset account = ‘inventory – cars’
        COGS account = ‘inventory car costs’
        Variance account = ‘inventory car costs’

    Under the ‘build’ tab,
        build one item
        add all the inventory items from the report, with quantities used
        set additional costs to match the ‘vehicle resale costs’ on the report, recorded to the ‘vehicle resale costs’ account
            this will credit the vehicle resale costs, resulting in a net 0$ balance

under inventory & services, build vehicle {vin} from item assembly
    build from bill of materials
    item to build = vehicle {vin}
    quantity = 1
 
When you sell the vehicle {vin}, record the project (car {vin})

The project income report will show all revenue and expenses.