This is a relatively simple, and quick suggestion, but amongst a few other items I think should be implemented into the shop such as the useless mob drobs and misc items that you get from McMMO, I think Soul Sand needs to be implemented.
I don't know about anyone else, but I see Soul Sand as the Gravel of the Nether, with the Netherrack being the Dirt/Stone equivalent.
Also, Soul Sand is present in certain custom biomes in the Wild, but as of yet is useless.
I can't imagine it would be too hard to place into the shop, and I for one would like to see it as the same price as Gravel. Just a thought.