If I understand correctly, the building won't have a floor, and the post bottoms and sills (if you have them) will be exposed to the weather. Why do you need the sills at all? Why not just pour a little concrete in sonotubes, and connect your posts to the concrete with some of those simpson post bases that drain? I can't recall whether you had 6 or 8 posts in your shed, but it seems to me that it would be real simple to pour these and make yourself a solid footing/foundation.
