I would start by contacting 3d.sk and asking them how far they think their conditions go, i.e. if they are supposed to be binding on purchasers of material made by another party using their sources. If that were the case then it seems to me that merchants would need to make customers aware of that.

Given that a merchant may use various purchased resources in an item, it could be a real nightmare if all of the various possible restrictions mandated by all the resource vendors are applicable to the final product. You could in theory even end up with conditions that directly contradicted each other.

If 3d.sk wants to insist that their terms transfer to anything created with their material then things would get very interesting indeed. I think they'd be foolish to do so, even if it were legal, but that never stopped anyone. I can see Daz's position with the AMG 'digital clone,' that was a work designed to mimic an identifiable individual as closely as possible. Even then, IIRC, there was such a furor that they eventually modified the terms of use.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken