The interest that House Kundarak charges for loans is responsible for the lion’s share of the house’s income. Its banks can be found in cities across the Five Nations and eastern Khorvaire— all magically warded and watched over by house guards. The Banking Guild regulates commerce in gem cutting and jewelry manufacture, money changing, and moneylending. It also advises (some would say dictates to) national governments about consistent standards for coinage. It works closely with House Sivis on accounting matters and with House Cannith on price stability for basic goods.
House Kundarak’s vaults are magically linked across Khorvaire and even to the city of Stormreach in Xen’drik. A customer who rents a safe-deposit vault can access that vault from any Kundarak bank.
Source: Eberron Campaign Guide
The Banking Guild oversees all major banking and finance across civilized Khorvaire, particularly in the Five Nations and the Lhazaar Principalities. Professions related to these services are the purview of the guild, which licenses bankers, coinmakers, gemcutters, goldsmiths, jewelers, moneychangers, moneylenders, treasurers, and similar professionals. It also works closely with the other guilds (particularly those of Cannith) to ensure fair standardization and stable pricing on goods and services.
Source: Dragonmarked