I don't know, Kern. I'd be willing to sell gravel, saplings and other useless guff that you end up hoarding by the thousands, and then use the money to buy more useful stuff like iron and foods. And with the new coins, you wouldn't need currency mods in multiplayer servers!
Although I think the Testificates shouldn't sell uncommon stuff like diamonds, ender pearls, and potion ingredients you get from the nether. Hunting for these rarer items is one of the thrills of Survival mode.
And who knows, maybe in the future we'll have other kinds of testificates besides general merchants. Apparently the color of a testificate's robes or apron dictates its profession. Maybe the villages will have actual blacksmiths, butchers and, uh... prostitutes?

