for Unreal , making fbx sometimes doesnt work. You may rig the models with Auto Rig Pro, or manually the same skeleton with Unreal mannequin. I can help you about it (not rig but creating an unreal project). So you can sell the package
I made the Animations in Unity, so I'd have to find a way to export it from Unity to Unreal and FBX is the only way I could think of.
I see, if one day you will change your mind and make your fbx is ready for unreal , please let me know, I can help with that.
Thank You for the offer, I'll let you know if I need your help.