1) They're (probably) free
2) [See Soundguy above]
Actually my dad built several sheds using creosote poles for the uprights and PT wood for beams, etc. For the most part, the cut portions of the poles (notches for beams, etc.) are still looking great after almost 30 years. I think that you would be fine for many years to come with an angle cut on top, but a small investment in a sealer will pay dividends in the future no matter what you use for your posts.
On a personal note, if you elect to work with creosote, be extra careful to avoid getting splinters. That will be the most painful splinter you will ever get. Trust me.