This is a great question! You may have noticed that aside from the Gree themed items returning to the store, that the items which are on sale sometimes changes as well. I recommend folks check the store every day to see what things are on sale. If we add any brand new items into the Market we will be sure to alert you, but the sales will be changing "on-the-fly."
I love how whenever the issue of Cartel Coins being spent comes up the Devs answer right away