The store has 10 random items that can be bought. These items can be Weapons, Equipment or Attachments. Weapons can be found in all Item Statuses except Prototype (that are bought in Workshop). Those statuses are: Normal, Broken, Refined, Flawless and Custom. Like in the Slot Machine, it is extremely rare for a custom weapon to appear in the normal store. Custom weapons seem to be more likely to appear in the store the higher your heroes are.
If any item is bought it will go to your Inventory. The empty place will be replaced by a new random item after next victory. If you don't buy anything, the store will be refreshed with 2 new items.
When any Equipment or Attachment item is purchased, one item from the last full line of 2 items is automatically sold. but the item you purchased doesn't disappear from the shop. This way you can buy multiple items of one sort and they will all go to your inventory.
Always check the full pattern before buying Attachments or Equipment, because you can accidentaly sell a valuable weapon automatically. This can sometimes be useful, because you can fill your inventory with multiple items and equip your heroes with it.