Layers is what I said in the first place but it does not matter if the layer is in PSD, PNG, jpeg, tiff or what ever. A layer is a layer and if someone wants to rip it off, file type does not matter.

I think Phil was asking if PSD was usable for a larger variety of users. No need for merchants to limit the buyer base. As you say a base texture and bump/displacement map and PNG masking/image would work in all my apps.