In this guide:

To add a courtesy car to the system, add the car as you would do for a normal stock item except specify it's asset class as 'Courtesy Vehicle' when adding the vehicle.
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If you want to change an existing vehicle to a courtesy car first find the vehicle, then click on the edit button:

Then click on the 'Pricing' option:

Then change the 'asset class' option to 'Courtesy Vehicle' and the market to 'None' to remove it from sale, then click save.

To convert a vehicle back to a retail or trade vehicle follow the steps above, except select the market as 'Retail' or 'Trade' and the 'asset class' to 'Stock',