Gorell the Quartermaster is located by the stairs in the stronghold. He sells potions and can repair your items.
He is also responsible for selling upgrades for the Staff of Herding.
Do not end your game after killing Diablo (Hell mode through Inferno), because a merchant named Gorrell will sell you the upgrade plans for the Staff of Herding (In Nightmare difficulty or beyond). Just portal back to town after killing Diablo each time for each difficulty mode.