Hi there,
Thanks for reaching out.
If the model is under Royalty-Free license, you can use it as long as it is incorporated into the product and as long as the 3rd party cannot retrieve it on its own in both digital and physical form.
You cannot resell the model you bought in its digital form and you cannot resell it in its printed form as a separate, single item. Let's say you buy a printable bust of a celebrity, print it and sell it - it would be considered as a copyright infringement. But if you'd buy a figurine for a chess game and sell the whole game together, it'd be fine.
We also always suggest buyers discuss the usage with the seller and gain their approval, this would make you a hundred percent sure that you can proceed with your decision of using the purchase or if you want to make sure you are using licensing correctly please contact our support team at support@cgtrader.com.
Kindly,
Austeja from CGTrader