I have seen (and have a few) that are marked as “This item may not be Property Released”
Linking to this info page: http://graphicriver.net/licenses/faq#property-release-a
If you are creating for example, a Phone – and using an actual Phone for Reference – you could just alter the design of the buttons and interface, et. – by creating a completely new phone ( ? ).
As you said – doing a Ferrari in all it’s curves, etc. could lead to having the item marked as “Not Property Released”
I would suggest that you review other mockups and do something that isn’t being done yet.
I know that sounds like work – but to stand out you’ve got to be different.
I work with Mockups and I’m still thinking about my next item – and it’s taking me some time.
But, you are creating 3D images for your items, it seems like the way to go though, combined with a Smart Object for users to place their content.