I want to start selling 3D models, but want to make sure that my ideas are not stolen.

Good question, my answer would be to sell the model as "Editorial" meaning it can't be used to advertise or sell a product such as a game - I am only saying this because that is how the photo licencing works on Istock etc.

Maybe mail Mantas or one of the guys at support?

Would love to see the models though!

You automatically are protected by copyright laws when you create something.
In general you do not need to do anything to have full copyright ownership on the things you create.
You also have full control and legal rights to make the rules and set limitations on how your work should be used (Note that CGtrader has an option to upload custom license so you can write your own user agreement and upload it if you like).

However , reinforcing copyrights is another story as the laws can vary in different county's.

If you really insist on protecting your intellectual properties, then you best gather and register evidence that you are in fact the sole creator of the IP, so that you can make a case when it would come to a disputes and let a judge decide hoe is on the right side (e.g. dates of submitting a work, drawings, vault services, etc.).

Note that copyright and the internet are things that do not get along that well.
Keep in mind that if you ad something to the web it will be copied whether you like it or not.
The thing is that most artist do not rely care about it to mush, basically you would just like to reach out and present the work to the people hoe have no problems paying for your work (CGtrader is good place for this).

Regarding the images you used;
In practice, if the images are altered and incorporated into a work and do not resemble the same appearance as the original image afterwards, then this would be considered a new work.

Thanks for the advice.

If you create an existing real-life object, then obviously anybody else can also model the same object and sell it, just as you. You're talking about your personal mesh and textures here, and those obviously belong to you as long as you don't pass on the rights elsewhere.
If you had to process out the textures from online images I guess it is fine, unless we're talking about branded logotypes.
Textures may be sold by texture makers, then one has to check the legal agreement on their site. Reselling textures one bought, even within a model package, is a nontrivial issue.

One of cgtrader.com member named joey22man_1 stealing my 3d model and sale it for $19.99
https://www.cgtrader.com/3d-models/vehicle/other/batmobile-tumbler#tab-comments

I share the model on http://www.blendswap.com/ for free download, after I come across joey22man_1 stealing the model I remove the model from blendswap.