Hi Alessio,
Once your model is sold, you get your money directly to your PayPal or Skrill account. You don't need to pay additional cash to get your earned money.
You can find more information about the payments here:
https://www.cgtrader.com/blog/everything-you-need-to-know-about-the-new-cgtrader-s-centralized-payment-system
All the best,
Eduardas