I only want to use the FBX models, not the materials. Just the geometry. It should be possible to buy an asset over at the Unity Store and import the fbx without material into UE and assign new materials in UE4, right?
Andrzej Koloska has a good tutorial on this: Prototype - Sci Fi Environment Import
You check out his complete tutorial: Shooter Game Tutorial
EDIT: His blog has moved to a new location: ://shootertutorial/
EDIT of the EDIT: Moved the thread to a different location as it is not really related to the Marketplace.
You just have to import the fbx files + textures into the UE4 - after that create a material with the texture and assign it to the mesh. 
This is very much possible, I personally downloaded the viking village demo and implemented all it’s meshes into a test build of mine.
If i remember correctly you just export out from unity in fbx format. However, now that im looking at my viking village folder inside the corresponding unity project folder.
I can see that all assest/models/buildings files are in .fbx and .fbx.meta extensions.
So you should be able to just import straight out of the project folder.
Is this completely legal for us to do? Purchase assets from the unity store and then use them how we please inside ue4 for our games/projects? I can’t find a definitive answer.
This is the best I could find: ://forum.unity3d/threads/using-of-unity-assets-store-in-unreal-engine-4.328865/
As far as I read it’s legal to use them in the UE4 (or any other engine) -> ://forum.unity3d/threads/asset-store-items-used-outside-of-unity.308535/