Would it be possible to use certain assets from the Marketplace (plugins or models/anims, which I purchased) for a mod for RR ?
No because you would be redistributing the raw assets which is against the terms of the marketplace.
Ok so after looking into this further I think its ok if you release a cooked mod aka the .robo file. but any kind of content where they have access to the raw files is a no go.
Usually so long as the content is in cooked format, you should be good. But I’d probably check with the Epic staff since they had a different stance on using UE4 Marketplace content in Unreal Tournament mods. (They said it can’t be used).