If they are to sell the CD at the show, I think they should sell it after the band start playing so you don't have to worry about getting the case and disc bashed during the show - and make sure the sales get counted in any chart runs.
I wonder if, once the album is out, the band will do a Pearl Jam style live release of each show? It seems fairly clear there are - or were at one recent point - plans for a "ChineseDemocracyTour" DVD? that presumably will be out after the album.
It would make more sense to sell it after the show is completely over as people are exiting. Not that any of this is really going to happen IMO but if it was sold during the show there would be mass chaos no matter if the band had already come on or not.