I don't think it's legal to resell these templates outside the Envato (themeforest) Market under any circumstances. OP may refer to the GPL license but almost all products from themeforest are sold under a Envato default split license. This means that components created by the author (such as CSS, images, graphics, design, photos, etc.) are covered by the Envato Market license which does not allow resale. In addition, this type of purchase will certainly not allow for template updates or author support.
WordPress is open-source software, and I don't think it's even illegal if use some templates on two or more websites. I have already had experience with this type of resale, that's why I'm asking.
I understand what you mean here that's why I tried to explain 'split licensing' from Envato market. WordPress is open source, but my design is not. Regardless of the GPL license of the WordPress platform, I, as the theme's author, have the right to protect my intellectual property. Therefore 'split licensing' comes into play.
