The method that @Lurch mentions would allow you to basically copy the purchase invoice but as a sales invoice (all the details - the nominal code, the description, the price, the VAT etc. would all copy over). Using the re-billing feature, you can also select to add a markup if you wish:
And then add your mark up (or leave it as it is, and edit the value yourself):

This feature is really intended for things like expenses incurred on behalf of a client, or while working for a client (such as travel, hotels etc.). However, you could do it like this if you wish.
As you mention you’re selling a car, and you’re doing this over and over again, I’m assuming you’re a car dealership of some sort? If so, this could be useful. For example, if you entered the purchase including mileage, previous owners, colour, model, then it would save you entering them twice.
Using a project tag could also help associate the purchase invoice and the sales invoice together if you wanted to do that, and that would allow you to see the profit on a car by car basis. You could even tag purchase invoices for new parts or work you’ve had done on the vehicle to the project tag too, to give you a full picture of the cost of the vehicle. You could use a tag such as the reg number (AA00AAA) or similar.
However, that all said, as @ian_roberts mentions above, stock is generally recorded as General Purchases on purchase invoices rather than as assets. The only time you could them as assets is at year end or year start, where they become “stock” (either opening or closing). There’s quite a few threads on the forums on this topic - do a quick search on recording stock, and you should find one that’s suitable.
Hope that helps.