The easiest way to build a movable shed, is to put the framework on a couple of large railroad ties(6x6 at least). They are almost impervious to moisture. My buildings are over 25 years old and they are still on their original railroad ties. I have two outbuildings built that way, and my tractor(42' Farmall) has no problem dragging them around. The building will settle, but what I have found to be the best way to move it, would be to cut a 45 degree angle out of the bottom of the railroad tie. This helps the building ride over the lawn without digging in to much. It also alows you to pull it out without resorting to a whole bunch of digging after the building has settled.
One thing that I would advise, is you are better off using three sets of timbers on the ground on a building that size. I would put them on the shortest side(the 12' side) as well. Putting up the building this way allows it to be moved whenever you want to. We wind up moving ours every five years or so. Usually when our stock fence is moved. One of the buildings is a 3 sided shelter for whatever is in the field.