It is hard to confirm how many of that item (especially cooking ingredients) you have in your bank when you are being offered something to buy at a vendor. You will have to need to relog into a character in front of the bank to find where the item is in your collection, then check the quantity, then log back to decide how many to buy it.
It will be very helpful to show “xx in bank” on the tooltip of a vendor item. Some items are only available at specific vendors who are far away from the cities and you definitely want to check it out before you decide how many you should buy.