is allowed under the UE4 EULA.
Either you can sell their mods with their permission (and you pay the 5% royalty due on your revenue), or you can allow them to sell their mods directly under the UE4 EULA governing their use of Epic’s code/tools (which requires them to pay the 5% royalty).
Does that make sense? We’re really excited about the prospect of moddable UE4 games and want to make scenario as practical as possible.