Does buying CGTrader car models create a royalty problem?

First you didn't mentioned what kind of cars, is it real world existing cars or some fictional/sci fi cars ?
Any car model (it doesn't matter if it's under editorial or royalty free license) that is exact replica of real life car can't be used in commercial game without premission of the copyright holder, car manufacturer. In other words, if you want Audi in your game you need to contact Audi and seek premission to use their name, logo and design in your commercial game, for everithing else only use models that are under royalty free license.

For royalty free models you should search for royalty free licence - it is theory.
And now practise - not every model has correctly set licence. And some artists (especially some big sellers on other maketplace) for example remove Ford, Mrecedes etc. logo and sell "generic" royalty free car models.
Then it is only up to you to recognize wheather you are buying true generic (not branded) model which you can use commercially without any problems and wheather you are buying branded models (with copyrighted design).

So if you are buying real car model (Mercedes, Škoda, VW) these models shouldn't be considered as royalty free regardless which licence is shown on product page from 3D designer. And theoretically you should ask for permission to use it from car manufacturer. Although it depend how famous your game is (or will be), if it not big project maybe you don't have to care about it so strictly.

theoretically also in this case you should ask Nissan before using this model in commercial project (removing logo doesn't make generic model, copyrighted is the whole car design), but on the other site I would say that nobody from Nissan will care if you use this model in your game. The worst scenario is that they would contact you and ask to remove this car from your game (it is just my idea, I have no official information). But especially if your game will not become worldwide bestseller with millions of sales (downloads) you don't have to worry about it - it is older car model and maybe also in famous GTA game they are also using "generic" models similar to real ones.

yes, you should be allowed to use them, it is the basic idea of royalty free licence )))

As previous members already stated, yes, Royalty Free is the way to go, however, if you use a car that is similar to a real one and that car manufacturer finds out about it being used in your game without their permission, the manufacturer can sue you. The amount you can get sued for can be much greater than you actually made selling the game. Use cars that can't be pegged as a certain model or with details that are vague or common to a lot of manufacturers.

If model is sold under royalty free license that doesn't necessarily mean that you can use that model commercially, a lot of peoples here doesn't understand how license system works and always wrote answers in theoretical matter. For example this https://www.cgtrader.com/3d-models/car/sport/ferrari-458-italia-df3e9aff-6ea8-4ecf-abb4-b10a4e3a246c is selling under royalty free license, so what you think can you use it in commercial way without asking Ferrari for premission ? Of course you can if you have couple milions of dollars that you don't know what to do with. As far as real cars goes: Name, logo, whole design and every single part of the vehicle is protected by copyright/patent law. You can remove logos and name, but you still can/will be sued for using copyrighted/patented design without premission, altering existing design will also get you sued depending on how much of the original design is in it, there is a defined percentage how much of the original design needs to be in altered one, in order for lawsuit to get finalized.

no