Two observations:
On the shuttle between Liverpool, Manchester and Leeds (SH060 referred to), 2 types of boarding passengers can be observed - those with fixed departure times and those with 'carnet' tickets. Two queues formed at the former Liverpool Coach Station - the fixed time passengers are boarded first, then the flexible ticket holders, presumably regular commuters, subject to space being available. A passenger once wished to purchase a new 'carnet' ticket but the driver didn't have a stock of tickets and the Liverpool ticket office had closed. She was carried to Manchester and was able to buy there.
Secondly, despite NX policy and publicity (and licensing regulations) promising that walk up tickets can be purchased, on a number of occasions I have seen passengers turned away, the driver resolutely refusing to entertain any notion of buying at the door - despite the coach be lightly loaded (and remaining so throughout its journey). I would never rely on being able to buy on departure!