I am thinking to buy a second hand yaris, was wondering average fuel consumption for city driving.
It's been well kept, low mileage and only 5ish years old.

Remember average fuel consumption is based on a new engine that has been properly maintained by the owner to the strict guidelines set forth in the owners kit.
It is a guideline and probably will never get that kind of mileage again. Toyota, along with other makers, use one type of fuel for those estimates and use strict controlled conditions.