At the moment, we do not modify the uploaded data when a model is downloadable i.e. the archive you download should contain the originally uploaded model as well as textures added in the editor.
If the user did not upload new textures and kept the same texture names, I expect that you should have the same rendering as on sketchfab. I'm just not sure this really is a matter of the format you download.
In the future, we might offer a way to "bake" the materials defined in the editor in the downloaded model but that's a tough subject (especially since no format really support PBR materials natively at the moment, maybe gltf will fix this).
If you need further help on this, please provide some links so that we can have a more detailed look!