The Sell Creep has a chance to appear in any normal shop. If items or guns are dropped on the grate, the Sell Creep will take them and give in return. The amount of given in exchange is approximately 45% of the base cost of the item.

Note: Active Items can be dropped by holding G or up on the D-pad, and guns by holding F or down on the D-pad. Passive Items like Junk can be dropped using the map interface.

Shops appear on every floor. Every shop has 3 random pickups for sale, usually along with several items, guns, or additional pickups. There is a chance that the door to the shop will be locked.

Shops have a chance to spawn another vendor NPC, Munch-er ( the actual name is censored on steam for some reason???), or the Sell Creep.

If the Prime Primer has not been bought yet, the shop in the Gungeon Proper will always have a separate display that sells it for 110 shells.

The Forge does not have a normal shop, with the Blacksmith selling items instead.

Repeatedly firing any weapon within a shop will cause the shopkeeper to first give the player a warning, then double prices, and finally become enraged and fire at the player. Afterwards, all shops will no longer have anything for sale.

If the player is killed by the shopkeeper, the death screen will say the player was killed by justice.

If the player survives, the shopkeeper will teleport the player to another room.

This merchant sells a variety of "realistic"-themed items.

Upon spending enough money on this merchant, he will move up to The Breach where he will allow players to unlock new weapons. He will still continue to appear in the Gungeon below.

Starts appearing after being saved from a cell in the Gungeon Proper.

This merchant will sell blanks and blank-related items, such as ammolets and the Elder Blank.

Starts appearing after being saved from a cell in the Gungeon Proper.

This merchant sells various magic items, charming guns, and heart containers. However, all items or guns purchased from this merchant will increase the player's curse.

Starts appearing after being saved from a cell in the Gungeon Proper.

This merchant sells various goop or fluid-related items, such as potions.

Upon spending enough money on this merchant, it will move up to The Breach where it will allow players to unlock new weapons. It will still continue to appear in the Gungeon below.

Having a sponge translates their language to yours so you can see what they are saying.

A merchant from the race of the lock-adora.

Starts appearing after being saved from a cell in the Gungeon Proper.

This merchant primarily sells chest-related items. Unlike other merchants, he takes keys instead of money.

If the player takes a shortcut to the third chamber onwards, the Resourceful Rat will appear in the elevator room and offer the player three free guns.

In the Black Powder Mine, one of the three guns can be taken.
In the Hollow, two of the three guns can be taken.
In the Forge, all three guns can be taken.

Shop prices depend on the quality of the item:

items cost 120
items cost 90
items cost 65
items cost 45
items cost 35
Ammo and Heart cost 30
Armor and Key cost 25
Blank and Half Heart cost 20
The exception to this is Table Tech Money, which costs 50

Shop prices increase as the player progresses further into the Gungeon, and each floor has a price multiplier:

Keep of the Lead Lord: x1
Gungeon Proper and Oubliette: x1.2
Black Powder Mine and Abbey of the True Gun: x1.4
Hollow: x1.5
Forge: x1.6
Bullet Hell: x2

Several items/guns can be used to steal items from shops, which will increase the player's curse by 1 per item. These are:
Aged Bell
Box
Chaff Grenade
Charm Horn
Decoy
Explosive Decoy
Grappling Hook
Grey Mauser
Smoke Bomb
Ring of Ethereal Form

Chances:
Stealing from any NPC will always work the first time.
For the main shopkeeper, the second theft has a 50% chance to succeed, and any further thefts are extremely unlikely to succeed. The main shopkeeper also remembers the player between floors, and the chance to successfully steal something increases each floor.
If the player is caught stealing by the main shopkeeper, the main shop will be closed for the rest of the run.
For other merchants, any theft past the first are extremely unlikely to succeed. However, these merchants do not remember the player between floors.
If the player is caught stealing by a merchant, that specific merchant will be closed. The merchant will still be open if encountered in a different location or floor.
Stealing using the Grappling Hook is guaranteed to work, but the player is also guaranteed to get caught.

Notes:
After stealing something, certain NPCs may refer to the player as "Thief"