Lets clear some things first (because of the other people that might have the same problem) there are two questions: 1. Are we talking about someone who downloaded/stole your 3d model and modified it in order to sell it as their own ? Or are we talking about someone who saw render/picture of your model and made model based on that render. For first thing we (community here) and cgt support need proof that model is indeed made by you wich mean that you have to provide for example full max scene with all blueprints used for building model in question (even better if blueprints are concept art made by you), material setup, layered psd for textures etc. Thief can't have those things, the only way thief can get those things is if artist lost his mind and made source files of the model available to public/buyers. From my personal experience cgt support take things like this very seriously and if you provide sufficient proof i'm sure that things like this will be resolved very quickly. As far as second thing goes, you need registered design patent in order to claim that someone copied your design. No one will take as granted any claim about design that isn't registered because anyone can claim anything. Your models and models of the person that you are claiming stole it from you wont match in any technical way wich mean that models aren't stolen, wich is the only thing that matter to the cgt support if there isn't registered design patent. Don't get me wrong, but in this case, the "other" side can also claim that you copied their design. Since you claim that is custom design made by you, you should be able to provide some concept arts, or something similar wich can proove that design is indeed yours, In plain words, 3d models are 3d models , design is design. Apples and oranges. Good luck.