You can use the products I sell in the marketplace because I have the right to sell them in the first place:  I designed and created them and own the rights.  That is different from the design taken from someone else's work:  the designs for the Alien vehicle are not (likely) the property of the seller and he therefore doesn't have the right to sell them.  Therefore you may not use them.  Just because I didn't steal your car stereo myself but bought it from a fence, I do not have the right to it -- its stolen property.