There is not currently a means to texture dice, although it is a planned feature. As such, any custom dice in a game would have to be images in a rollable table, so the best way to sell them would probably be as image packs. I would recommend selling a full basic set (d4, d6, d8, d10, d12, d20, d%), which would be 70 faces (since the ones digit of the d% is the same as the d10).