Have your models been stolen and sold on other websites by anyone else?

What do you want to say? Someone steals the models from the others and are selling as their own? That's totally unheard of.

I already have it. Just at the end of last month, I and many artists I know had a series of models stolen and sold on Chinese websites. We simultaneously contacted the admins of those websites and succeeded in getting them to take them down. We cannot control the entire internet, but we can prevent them from being sold illegally on high-traffic platforms.

what websites do you mean, I wonder if my models are there or not?

It happened to me. I had it removed from the site via DMCA application.

To all artists having models stolen: do not hesitate to file DMCA like @xantier - it works most of the times, because it can be used as a base for legal case which can cost the offender up to $100,000 per case - if he steals models from multiple artists (which is usually the case) this can lead to a pretty serious sum and is great opportunity for a class action lawsuit (https://en.wikipedia.org/wiki/Class_action) where one person can represent a group of offended artists.
Amongst other advantages this can also help reduce the initial cost for opening the case for individual artists. So it's best to contact some of the other artists that got robbed by the same offender and coordinate.

As a first step, you can write to the site owner (if there is contact info on the site).
Write your email in a professional and cold manner (do not insult or let your emotions take over).
Write the links to your infringed models on his website and add links to the original models on CGTrader (or wherever they are) and include:
"You neither asked for nor received permission to use my work, nor to make or distribute copies of it. Therefore, I believe you have willfully infringed my rights under 17 USC Section 101, et seq. and could be liable for statutory damages as high as $100,000."
+
"remove them and destroy all copies immediately, and desist from this or any other infringement of my rights in the future. If I have not received an affirmative response from you by close of business (5 pm Eastern Standard Time) on /////Add Date 24 hours after you send the email///// indicating that you have fully complied with these requirements, I shall consider taking the full legal remedies available to rectify this situation. "

This e-mail will also give you leverage and can be used as proof in a future lawsuit.

If the site owner does not reply, use
https://www.whoishostingthis.com/
to find the hosting provider of the site and send the DMCA to them (most of the hosting provider sites have a direct DMCA form) and they may even take the whole site down if they receive valid proof (some also temporarily disable the site while they investigate). Just look for a "Report Abuse" or something similar on the hosting provider site main page (it's usually at the bottom) and then "Copyright/DMCA" report.

I hope this helps.

Hello. The DMCA application will start with you completely filling out the form in the link I wrote below. In this form, they will ask for information such as your personal information, the correct address of your works and stolen addresses. The process takes a very short time. It is completed in almost 2 to 3 business days.

I forgot the link. :)
https://dmca.lindenlab.com

Removal of your works from the site.

The situation I am experiencing is generally caused by the users, not the website. Because the website is a market, people can be misled into thinking that their works belong to them.
I'm 'marketplace.secondlife.' I had a problem with . And I even saw that the same person, a work owner I know, was selling his works there, and I reported it to the owner. It was the 'B-Made' user in this market who stole my works. And I believe that all the works of the user 'B-Made' currently on the market belong to others.

The problem with DMCA is that is effective only when the marketplace itself is legit, but when your models are being shared/sold in warez site that is registered in country that does not respect DMCA, then you're out of luck.

what can we do about that ? is it possible to protect our models ?

If your models have been stolen and sold on other websites, consider the following steps:

Report to the Platform: Inform the website where the model is being sold and file a copyright infringement complaint with them.
Legal Action: Consult with a lawyer to explore copyright protection and take legal steps.
Watermark/Signatures: Use visible or invisible watermarking so as to deter illegal networking.
Monitor online marketplaces: Use services to track and defend your digital assets.
Intellectual property has to be kept safe in the digital space.