It all depends on Epic. Marketplace is their store, not mine, I don’t make the rules. If they will allow discounts or support discount codes, then sure. BUT… the animations you already have from Unity WILL WORK in Unreal. They just have a different skeleton (standard Motionbuilder skeleton). Skin your character to it and there you go.
I guess 50% of assets sold here, will be assets you already bought on Unity Asset Store, so this will be a very common case.