I'd pay good money to watch fifty people braying on the guard's cab door demanding to be sold a ticket immediately. I'd pay even more money to watch the guard only open the local door at each station and check and sell tickets before departure.
After all, Northern have clearly stated they are now the rules.
I've not had fifty, but I've had a good number queuing at my back cab door on a 313 waiting to buy tickets. A good example is London Rd (2 mins) Moulsecoombe (3 mins) Falmer. I've never sent anyone away, only rule keep out the cab. I've said it before, we get folks who come to you and buy, and others who see you, head to the other end and hold a £20 praying you wont make it to them. BTW. These are on lines that all have a ticket machine and many with staffed ticket offices.