Not sure if this has been requested before, or indeed, if it's even feasible.
But... would it be possible for each seller (or maybe seller+, or something), to be allocated a small amount of space on the site where they could upload files. Maybe even only certain types of files for security purposes. If this isn't feasible or practical, then alternatively, have the option to create custom pages which can be linked to from a listing, a sort of very basic content management module.
My reasoning is, if I take an example from myself. I sell dog harnesses and each individual harness has a sizing guide. It would be handy to be able to link to a document from each of the listings so that a potential buyer could follow the link to the guide and see what size is best suited to their dog. To do this currently would require me to link to an external site - which is forbidden, or to add pages and pages of text to the descriptions of all of the relevant listings.
This is only one example, but I suspect that there could be other applications.