When it comes to game mods in general, as long as the item is made by the end user (not taken from another game) and no money is made from it then it is no different then fan art.

 

In regards to name brand items in a market place, stores such as Turbosquid have been selling cars that are clearly marked as real world vehicles, 3 of which I have purchased.  But each was made by the artist, their models are original though they look the similar to the naked eye.  I am concerned about Lamborghini comming after me, no, not at all.  I paid for someones artwork.  How is it any different from someone taking a picture of the real car and selling the picture when the car company does not make a dime from it either?