Textures and prims, and now meshes get dated timestamps on them showing the moment they were uploaded, or rezzed, for prims, the first time. If a creator really has the issue of being copybotted the upload/creation/rezzed timestap should prove who created the item first. Requiring payement info would only reduce the amount of people selling legitimate stuff on sl. In turn reducing the amount of items purchased.
As far as inworld stores, they are disappearing. The marketplace is pushed "as the place to buy" in the browser, on dashboard, etc.. So much that many of the inworld stores i use to