Shop

In a Shop, players can buy between two and five Items from a hanged man. There is a chance that any item in the store will be 50% off. This is indicated by the price being red. Prices are:

3¢ for a regular heart, 2 at a discounted price.
5¢ for any other pick up (bombs, keys, soul hearts, pills, and cards), 3 at a discounted price.
15¢ for an activated or collectible item, 7 at a discounted price.

The shop always requires a key to enter. If the Secret Room is adjacent to it, it can also be accessed by bombing the wall between them. The Hermit card will take the player straight to the shop. The Hermit card also opens the door so the player can exit if they do not possess a key.

Greed (or Super Greed in Wrath of the Lamb) will sometimes appear in a shop, replacing the shop on that level even after defeat.

While in the shop, the track Greed plays in the background.

Shops can stock the following items:

Item	Icon	Effect
The Ladder		Allows Isaac to walk across gaps one square across by automatically placing a ladder between the two walkables.
The Battery		Gives Isaac an extra charge every 15 seconds per room, up to a maximum of 3 charges.
9 Volt		Leaves Isaac with two bars already charged after using an item. Upon pickup, Isaac's current activated item will be fully charged.
Steam Sale		Reduces all Shop prices by 50%, rounded down when an item, rounded up when a pickup.
Book Of Revelations		Adds 1 Soul Heart and makes the next boss a Horseman (except on XL Floors and on floors with fixed bosses such as Mom). Increases chances of a Devil Room spawning by 35%.
Treasure Map		Reveals all rooms for every floor, including the Secret Room. However, it does not reveal icons.
The Compass		Reveals all special rooms (Boss Rooms, Treasure Rooms, Shops, etc.), but not Secret Rooms/Super Secret Rooms or regular rooms.
Notched Axe		Breaks a single rock when activated.
Mom's Purse		Allows Isaac to hold 1 additional trinket.
Mom's Coin Purse		Spawns 4 random Pills around Isaac.
Habit		When Isaac is hit, 3 points of an item's charge are filled.
The Candle		Throws out a blue flame that can pass over gaps (but not through rocks) that damages anything in its path. Recharges 3 seconds after firing.
Portable Slot		Upon activation, 1 coin will be consumed, and a random Pick Up may drop, similar to the Slot Machine.
Bum Friend		A portable Beggar who follows Isaac, pick up coins for himself, and occasionally drop Pick Ups. Does not spawn Collectibles. Can push around dropped bombs, similar to Meat Boy.

Occasionally, a Shop may switch one item for another in a second visit. This may be a bug. (Red price items can be swapped immediately after completing the Challenge Room. Combining this with Steam Sale can effectively refresh your whole Shop inventory.)

Trivia[]

When asked on Formspring why the Shop room has a dead body in it, while the Sacrifice Room is empty, Edmund McMillen replied: "Because God works in mysterious ways."^[1]
Buying an item at discount price actually gives you half the sale price of the item back then subtracts the full price. This can be tested with Infinite Money and the Steam Sale.