You can use the store by examining the phone on the desk. It’s a normal store where you can buy items such as weapons, weapon parts and replenish your ammo and herbs. The Lvs. of the weapons is dependant to the character with the max Lv. The higher the Lv. the better the weapons you can buy, but note that the max Lv. for weapons is 95.
You can also sell your stuff for a REALLY low price and the best of all, you can purchase directly from the online store (Playstation, Xbox and PC). Note that everything available from the online store is available if you play Raid Mode long enough, with exception of the characters.