For those interested, I believe I've discovered the source of confusion with the customer.
On product pages, the informational pop-up regarding the license agreement states "Products published with this license may not be sold, given, or assigned to another person or entity in the form it is downloaded from the site, but can be used in your commercial projects multiple times after paying for it just once."
This clearly states that purchased products CAN be used in commercial projects, but does not offer any further details or limitations. This ambiguity led the customer to believe that they could resell 3d-printed copies of the downloaded files as-is.
Sections 21A.1 and 21A.2 in the General Terms and Conditions clarify that purchased products CANNOT be resold or redistributed as -is, either in digital or printed form. They further clarify that purchased products CAN be used commercially IF AND ONLY IF they are relatively small components within a much broader final product (referred to as "Incorporated Products").
21A.1. Product may not be sold, given, or assigned to another person or entity in the form it is downloaded from the Site or in 3D printed physical form. Product may not be used to create visual content, such as scenes, videos, or designs, which are published or sold through other stock media clearinghouses, unless it meets Incorporated Product criteria and it does not play a key role in the visual content.
21A.2. 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.
After speaking with the customer, the confusion is understandable. The informational pop-up on product pages seems to imply permissions for commercial use that the General Terms and Conditions strictly prohibit.
I believe this presents and opportunity for the CGTrader support team to update the text presented in that information pop-up to prevent future misunderstandings with customers.