on stolen models / very crazy idea

It could be possible for certain file formats like OBJ where vertex order is recorded as text, but I'm not sure how that might work in a native binary file format, like .max or something else. What about models which were created from parametric primitives in the same software package. Let's assume theoretically that a bunch of people are selling a doughnut created from a simple torus primitive in 3dsmax. The only distinguishing factor between those products is the texture maps. The vertex order on all those models "could" theoretically match up perfectly. So it would need to consider more than just vertex position and UV coordinates. All models created with parametric primitives could contain the exact same vertex order and UV coordinates. I'm not saying those are good products, but I'm sure they are out there.

Actually in topology, the size or position of the object does not matter. Two cubes of different sizes, with 8 vertices each, placed at different spatial coordinates, are topologically identical. Nor does the shape of the object matter. If we take the upper face of one of the cubes and make it smaller, to form a truncated pyramid, they will continue to be topologically identical, even if geometrically they are different figures. And the opposite also happens, we can have 2 cubes exactly the same geometrically and positioned in the same space, but be different topologically, if one of them has been subdivided.

Regardless of normals, what identifies a mesh is the number of vertices, faces, and which vertices are part of each face. If you rotate an edge, the number of vertices and faces remains the same, but in 2 faces one of the vertices that the shape has changed, would no longer be topologically identical, although the degree of coincidence would be high and it could be said that one comes from the other. There are many models with messy meshes, in which some edges have been removed or added here and there to try to hide coincidences with the original model. Another thing that can help identify a mesh is the poles. They also rig them (in the models that have it), because it is something that many people do not master and do not usually modify it. You could analyze the mesh information from the obj file, but it would be neither simple nor fast. Not all exporters order vertices in the same way. Maybe it would be easier to analyze the UVs (there is one less dimension to analyze) and the textures, but being realistic, I see that it is very unlikely that nobody would bother to do anything. The only ones that I can think of that could do something are the Blender developer team, some addons have been made at the request of users.

Something simpler, than CGT if I could do, is to force to display clear images of the mesh, the UVmap, the textures and the rig. Or at least the model information (number of vertices, faces ...), which is often left blank, for the same reason (hide the evidence).

If marketplaces would want to show initiative on identifying thieves, they could easily catch 80% of them in no time. Simple hash sum check would help to identify majority of stolen models. It's trivial task, yet it could be very effective. The problem is that no single marketplace would want to implement strict and effective measures against illegally sold content, because that would put them in disadvantage against their rivals. Effective fight against thieves would only be possible as coordinated effort. Not sure if that's ever possible though.

As far as I can tell, eventually 3D models will get part of smart contracts in blockchain technology, everything will be stored in the blockchain and be part of every transaction/smart contract.

Considering how many years it takes for TS to make just a new publisher page and how much time it takes for CGT to fix the site bugs, I don't think we will be alive to see that blockchain dream... I hope I am wrong..

As a stolen car may change its color, number etc, The same with stolen 3dmodel - retopology (many programs have autoretopo tools) will change relation between vertices...

Just compare checksum or SHA1 hash of the model's files, and see what's the same, or do an FC (file compare) from the command line which will spit out the differences (warning, FC is best used for files less than 1~3mb, otherwise you'll be in for a lot of beeping and pain).
Checksum or hashes are mathematical computations that are basically the sum of the entire file's digits added into one giant number. It's used by many industry leaders to let folks know they got a valid file in all it's pieces downloaded without anything 'extra' unwanted stuff getting it either.
IF a user changes the file, even a little bit, it'll look way different number-wise. There is no way to 100% guarantee success using this method, or it would have already been implemented, but it will stop a lot of automated scamming right out of the gate.

A few weird ideas that come to my mind.. in order of complexity.

I know that it's possible to autoconvert Maya, Blender and 3DS Max files automatically to other formats. "The other marketplace" does it, directly from the uploaded files. They are called "Generated gLTF". It can be done "under the hood" without making the 3d data public. At this point something could be implemented to compare the geometry of the new uploads to an internal database of the models that are already on Cgtrader, containing some sort of description of the model, maybe a convex hull or a voxel cage, to compare the general shape and proportions? At this point if they are found to be too close to other models they could be "flagged" for review before they are allowed to be released. Or the percentage of similarity could be shown in the model description.
It would involve a mixture of automatic calculation (to skim the amount of models to be checked) and then some manual work from moderators. Or in case the percentage it's shown in the model description, the community could spot the thieves more quickly.
Of course it's not an easy thing to do but it is possible. And of course it would only work on the files that are uploaded on Gctrader but it would be better than nothing...

Alternatively, the auto-conversion could be just used to "force" a render of the wireframes.

Or, since a 3D modeller with a minimum of experience should be able to export a model to obj or other formats, make this mandatory and again, implement an auto-render of the wireframe.