[angry_octet]

This is only a problem if you want gate-less tickets, i.e. get on a bus/train without scanning at a reader or network comms, which seems an overly strict requirement.

[anticensor]

Gateless ticket control is a must if you want to do a fast all-doors boarding in a bus.

[angry_octet]

And no comms between client device and internet endpoint for several minutes? Is so, you can say 'network down, manual scan required'. If client can get network, it buys a ticket online.

[Freak_NL]

Buses and trams typically don't have gates, but a common solution is to have the check-in device right inside the doors.

[anticensor]

That is only suitable for FDB.