To move inventory parts to a used car WIP you basically have to sell the parts to it, same with repair reconditioning labor if you have employees. Some costs add to your basis in the vehicle, some are just cost of sales.
Still valid after 3/4 of a century is the GM Accounting Manual as a guide to accounts and accounting for auto dealers. It is proprietary but easily found (gm.acctmanual.com) to view
Look at account 240 Used Vehicles to see what is added to inventory asset and examples of the sale of a vehicle. See account 446B for more on how to handle sale of used vehicles.
In a nutshell, the inventory value of a used vehicle is, the lower of trade-in allowance or wholesale appraisal or the purchase price if bought outright. Added is internal selling price of labor and materials for reconditioning and travel costs to acquire pkus any auction fees.
Costs such as state inspection, full tank of gas, "free" floor mats, final wax are not COGS but ordinary cost of sales
The NAADA, North American Auto Dealer Assoc , surely has publications on accounting for members, and every dealer small or large should belong