My most recent bulk purchases were diamond and obsidian (for repair and pixel-art).
I like your idea of 5 weekly deals
. Be sure to
read the signs look around the forum (such as these discussions
[1] [2]) for tips on managing a shop (like pricing, supplies, and such).
EDIT: And oh, if you want more ideas, take note of what people ask for in the global chat. Some of the things I've seen people buy are wool, brick (item), leather, dyes, and enchanted diamond stuff.