The store is a place where you may purchase new equipment:
This is also the place where you use your money gained during PvP and PvE. Mithril is also used here. To purchase certain items, a certain rank must be reached. Like for example, you can only buy the Black Disk when you're Rank 9. Every now and then there is a sale where items are discounted but only for a limited time, usually 12 to 36 hours.
Sale is a discount in the game. Sales sometimes occur during weekends, weekdays, or once per week. Sales help a player get the desired equipment, however, in the randomized matter.
Big Sales are sets of discounted equipments in the game. This shows when you got an in-game notification, either a weapon represented and shows the discount price. For example, you get a message "...Black Stars is in 25% OFF!...", then there are more sales had shown aside from the weapon represented.
Take note that sales only occur to Mithril weapons. Sales have never yet been on armor, backpacks, or things that cost gold.
IAP, or In-App Purchases, are purchases that made in-game to get specific currency or extras (e.g promotion, mithrils). Purchases can be made in the Bank. Bank can proceeded from the top menu, which is activated when clicking 'your rank' button.
IAP, however, has reported some bugs regarding of purchasing some purchases. Example of this is the unavailability to get the desired thing to get in a purchase, like Mithril.
Unfortunately, some IAPs are pretending to be hacked by some players in Star Warfare. This will cause your Game Center account to be unusable in Star Warfare, meaning you have to sign into Guest Mode, a mode that is not tied to your save file.