My thoughts about item stores are pretty clear: they suck in most cases. They are often full of the best stuff that is unobtainable anywhere else, and that is their primary marketing mechanic and their downfall. Profit is made because the items in an item store are unique and/or powerful. Just look at how many people have the celestial mount from the WoW item store? It has no impact on the game except that it looks cool, and Blizzard has made boatloads of money off of it.
So that is the recap, now for the idea. Item stores should sell only items obtainable in the game. Now maybe these items are rare, and maybe it will take a long time to get them, but its possible to obtain them through normal gameplay or quests. These items should all be able to be sold on the market/auction house, giving players an opportunity to turn cash into in-game currency legitimately. This would curtail the gold "black market."
Just a thought.