You can accept payment for mods if it is a personal agreement between you and the person commissioning the work, BUT only if it was created using open-source or fully licensed (commercial) software which allows you generate a profit, and includes no Bethesda or 'borrowed' assets from other modders work. So for example you can make a clothing mesh on commission using your licensed software, but only supply it as an un-rigged object with no Bethesda/borrowed body meshes, skeletons, assets or textures included. After you have delivered the mesh and accepted your payment for your work it is then up to yourself and the person who commissioned the work whether or not you both want to then share that mod for free or keep it for yourselves. If you wanted to make the mesh into a game ready asset you would have to do that pro bono, since it would then be necessary to use Bethesda/ borrowed assets.