Purchasing Agents
A purchasing agent is someone you hire to buy things for you when you are not around. This is typically to buy items that have a short shelf life, or are rare or popular. These people are generally agents of a bank in a city or large town; if you need a purchasing agent in a small town or village, you will have to find someone you trust, as there is seldom any sort of bank or financial organization in such small population centers.
To hire a purchasing agent, you pay them a monthly retainer, which they receive no matter how much or little work they do. This monthly amount is typically something along the lines of 50 gold a month; like any worker, paying more will tend to encourage them to work harder. They then get a percentage on top of that of how much money they spend for you to buy the items you want, typically 10%.