For keys used in Diablo II's Pandemonium Event, see Pandemonium Event.
For keys used in Diablo III's Infernal Machine event, see Infernal Machine.
Keys are needed to open locked chests in Diablo II, unless you are an Assassin. They can be purchased from NPCs in town in any act, or found outside of the towns. All keys are the same. They can be stacked up to 12 in an inventory square. Any size stack will simply be listed as "Key" on the ground, in the same way as Arrows or Bolts.
Merchant Trade[]
Cost 1 (Buy): 40 Gold
Cost 12 (Buy): 480 Gold
Cost 1 (Sell): 22 Gold
Cost 12 (Sell): 264 Gold