QB is accounting, are you a beginner at accounting or with QB, or both?
in QB, company menu select the home page, then use menu edit>preferences and select the desktop on the left, select show home page when opening on the right and save. The home page will show you the flow to some extent. You need to work with someone who knows QBDT to get you started.
You will also have to turn on inventory in preferences
Click chart of accounts and use the button bottom left to create new accounts
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create a dummy bank account called WIP (work in progress)
Purchase the used car and use that WIP account as the expense for the purchase.
Any thing you buy to repair the car is also posted to that WIP account.
If you use on hand inventory items for the repair to get it ready for sale, use inventory adjust, set the adjusting account to the WIP account and lower the qty of the items used.
That accumulates all costs for the car in the WIP account.
When the car is ready for sale:
1. Create the inventory item for the car in the products & services list
2. create (if you need to the first time you do this) a dummy vendor named in-house
3. Use the WIP bank account to "buy" the used car item you just created from the in-house vendor, the qty is one, and the total item cost is the total amount in the WIP account
The inventory item car is now in inventory with total costs to get it ready, and the WIP account is zero balance ready for the next car you need to do. You can have as many WIP accounts as you need if you are getting more than one ready at a time. ie WIP, WIP-1, WIP-2, etc
And of course you use the inventory item for the car you are selling on an invoice or sales receipt.
Any costs to keep the car running after it is ready for sale is repair cost expense and not part of item cost.