if a site accepted tiff files they could be layered. i don't know of any site that accepts layered files. none of the ones that are on the links on the left do in any case.
I remember a site that sold layered work being advertised on photoshoptv (which is downloadable through itunes for free) ... the site was called squoosh, or squirt, or liquid or something like that... not sure if they accepted files from everyone though..