Most Realtors don't charge an upfront fee. There are some exceptions.
As someone mentioned below, limited service companies often charge
upfront fees. Some full service Realtors will sometimes charge an upfront
fee to a client who wants extra advertising above and beyond. For example,
you want your executive home to appear in the NYT or WSJ real estate
section - that's typically an add on. But it's all negotiable.
Remember, as someone else said below, though Realtors are putting in many,
many hours for free and absorbing hard costs as well, most homes sell as full
service listings without any upfront fees or add ons.
Thanks for asking and contact me if you'd like to speak about your home,