Every citizen has an Oberin bank account. Your bank account is a totally secure place to store items. Items in your bank account can not be lost or stolen. Like your standard inventory, there is a limit to how many items you can store in your account. However, there is no weight limit for bank accounts.
Making deposits - walk into the bank and drop the items you want to deposit. Any items dropped in the bank will automatically be deposited into your account.
Making withdrawals - walk into the bank and choose "Bank" from the "View" menu (Command-N). A list of items in your account will be shown in the "Bank" window. Click on the up arrow next to the item you want to withdraw from your account. If there are multiple items present, like 10 heal potions, then the default is to withdraw all of them. Option clicking on the item's up arrow will allow you to enter the quantity that you want to withdraw.
Every major city has a branch of the Oberin bank in it. Items deposited in one branch may be withdrawn from any other branch.