The terms actually say you cant sell something in the form it was downloaded. It says you can sell it if its incorporated into something else. If you buy an stl of a dickie bow tie, print it out and sell it on etsy your breaking license. if you put the dickie bow tie on a doll then sell the doll on etsy you haven't broke it as you have incorporated the tie into a doll. If you downloaded a chess piece, printed it and sold on etsy your breaking the license. if you sell a chess set with the board etc. and included the piece you haven't broke the license as its incorporated into a model. problem is that sellers of stl just see the cant sell part of the terms and don't read or understand the rest of what it says . Anyone who doubts this can ring customer services and clarify for themselves like i did and many others have. I will put an extract from the terms below with link.
The Buyer’s license to Product in this paragraph is strictly limited to ******* Incorporated Product*******. Any use or republication, including sale or distribution of Product that is not Incorporated Product is strictly prohibited. For illustration, approved distribution or ********use of Product as Incorporated Product****** includes, but is not limited to - as part of a physical object such as a toy, doll, or model.
https://help.cgtrader.com/hc/en-us/articles/360015124437-Royalty-Free-License