If trying to sell original literature collectables I think being able to have the file directly attached to the sale would make it infinitely more rewarding than emailing someone to get the file.
PDF would be great for vectors as well. Right now I have to convert my PDF vector to SVG. The difference is that the PDF is 814 KB, and the SVG is 3.7 MB.
Also Augmented Reality + 3D animated assets. We develop WebAR and other kind of AR experiences and love to find some marketplace for them. Regards. Seba, from www.novaworks.com.ar