Depends on how well maintained the vehicle is kept, and on the driver.
US EPA lists:
http://fueleconomy.gov/feg/bymodel/1..._4Runner.shtml
depending on 2wd/4wd and the transmission, expect about 17mpg on average.
While the seller may be able to get 26mpg with his driving style, I would doubt that you would be able to do the same without getting some driving tips from the seller...