i haven't seen them at the festival before, or available online at all. i'd imagine they'd sell a ton if they ever had them for sale. this was mentioned in another thread, but at bumbershoot they sometimes have limited copies of sasquatch posters in the building where they have (or used to have) the ice skating rink in the winter.
No, they have not sold them at the festival in the past, and I assume they won't again. You can usually get them off the artists websites (usually not till after the festival) and as mentioned many are available at Bumbershoot festival. There is a poster exhibition called Flatstock and many of the artists (as well as a bunch of others) set up booths.