Changing texture on a placeable object?

Is it possible to create an inventory for textures/materials and apply those textures/materials in-game to a placeable object?

Do u know any tutorials about it? I couldnt find any

Yes it is

Make a Array of Materials. Get the Objects Static Mesh Component(s). Get Material from Array. Set Material.

How you get the Object refference depends on your Project (Overlap, Linetrace etc). Same for the Index for the Array (usually UMG Widget)

If you just want to change some Material Parameters look after Dynamic Material Instance in the Docs or Youtube its covered a 100 times already.