Don’t take this as legal advice. Since I am looking into this myself though, I will share what information I’ve found which suggests: Yes, you can make a moddable game with marketplace assets. Here is the reasoning as to why.
ARK Survival uses marketplace assets and allows modding through the Unreal Engine. I’ve never played ARK but watching a video how to make mods it looks like they use the following assets: Ultimate Rocks, Ancient Ruins, Wooden Props, etc.
I mention ARK because Epic specifically advertised modding for it on their launcher.
Essentially, the terms of service just don’t want you reselling the assets in a raw format. There isn’t really anything you can do from preventing those who really want them from obtaining them even in a packaged game. And in the case of a moddable game, your intention isn’t redistributing paid assets to other people, the intention is allowing a community to make mods using your game and Unreal.
And finally, even Tim Sweeney himself states an encouraged relationship between the Marketplace and Modding: Does this mean that games cant be moddable? - General Discussion - Unreal Engine Forums
If Epic is unhappy with what you do, I doubt they’ll take legal action. They only seem interested in doing this when a developer tries to sue them over their own Engine. Or in the case of the game Hatred, to separate their branding from the controversy. Most likely they’ll just politely ask you to stop, since they probably want to maintain the healthy and happy relationship with developers they’ve spent years building.