The shop doesn't actually sell gems: it only has a player stock, and used to only sell gems that another player has sold to it. This changed with the Personalised Shops update, where it only sells back to the player what the player sold to the shop. This makes the store very inefficient for buying or selling gems. It is not wise to sell gems to the store for it to "hold" because it will act like a normal shop and lower its stock on a regular basis, effectively removing your gems from the game. There is a spawn of two bronze arrows in the store, apparently left there by Brian.