Why not inform people that they are obliged to buy a ticket AFTER they have left the train if they have not got one before?
A straw poll of normals in the office (16 of them) and ONE of them knew you could buy a ticket at destination and were concerned that they could be "fined" as a result of, as they thought, not being able to purchase a ticket. (all but two knew you should not board without a ticket at manned stations because of the "criminal record" posters)
Even the NRE website just says :-
Where can I buy tickets?
Online - National Rail Enquiries do not currently sell tickets but you can purchase them from one of the Ticket Retailers shown after you have selected your ticket in our Journey Planner.
Over the phone, using one of the numbers shown in our Train Company pages
At a station Ticket Office - check our Station Information pages to find which stations have one, and what the opening hours are. Various forms of payment are accepted.
Using a 'self-service' ticket machine available at many stations.
Rail appointed travel agents are complementary to ticket office at stations and Train Company telesales service, and the majority provide a comprehensive travel service. Appointed travel agents can make reservations and issue tickets.
If there are no ticket purchasing facilities at the station (such as a booking office or a ticket machine, or they are closed/out of service), you may be able to buy your ticket on board the train.
Now, from the final paragraph, most reasonable people would not infer that buying on arrival was an option. Using the term "the station" is obviously referring to the departure station.
As I said in a previous post a simple "Tickets must be purchased at the arrival station if there has not been an opportunity to purchase before" would be enough, both added to the above and on the posters.
It may not technically be entrapment as some have pointed out, but it sure seems like it to me and many of the people I questioned. An attempt to trick ordinary travellers into unknowingly committing a criminal act, not to catch serial offenders at all.