Is there any way you could add a function to give the player an item (box.wooden.large) with a custom skin id that they are able to place, for serverowners wanting to sell via GUI shops?

Right now the cars are just spawning in front of players, and this could be used to grief other players, so with the solution of an item, you could do a auth check and return the item to the players inventory if they do not have auth.
Also players would be able to place the car where they want - we do already have minis and scrap helis for sale, where they get a box with right skin picture to place it where they want.

I know a function like this is possible, however I do not have the programming knowlege to pull this off.

Thank you in advance ^^

There are already plugins that do that, such as Portable Vehicles.

How is spawning cars in front of players being used for griefing? When using the mycar command, the plugin checks for whether players are in the way. Unfortunately, it's not feasible to do such checks for the api and server command usage, because shop plugins don't have a way to negotiate with the plugin whether the car should be spawned or not, so the plugin has to spawn the car anyway.

They can go insde other players home, go buy a car in GUI shop and POP goes a car inside players bases xD

Oh, did not know Portable Vehicles could do that. Thanks!