I don't think the store is really used anymore. But you used to be able to buy things like name glowing effects, and other useless idle fun items. Problem was, there tended to be more spam on the forum as people would made fairly useless posts to get ore to buy these useless items. (It was something like 5 ore per post). You could also store the money in the bank to get interest and buy interest upgrades, which always seemed to fall into the hands of the same people whenever the store was restocked.
The rank is basically just your post count.