I looked in to exactly what the legal position on this was, and I reached the conclusion that where they were transferred for fraudulent use one of or more of a sea of fraud related offences were comitted*, then found out that what TfL did was prosecute for theft as you are under a duty to return the ticket after use and by giving it away you commit theft*.
If you use the ticket you are travelling without a valid ticket and I would think the court would think you intened to evade the fare so they would convict of the more serious 1889 offence*.
OT, the law on contract requires consideration to pass both ways, which can be action, a service, not performing an action, transferring physical goods or a chose in action, or promising to do any of those. I am not aware of tickets being in any way related to bills of sale. My Contract law is a little rusty, but I think I'm right on this.
A return is sold for the use of one person both ways, so technically it is plain fare evasion to travel on the return half alone.
The main legal obstacle to the trade in tickets is byelaw 21 that provides (in part) that no person shall sell or buy any ticket or transfer or receive an unused or part used ticket intending for it to be used otherwise than in accordance with conditions. No offence is comitted where the sale is by an authorised person or ticket machine. If the ticket seller knows you are buying for someone else you are fully legal. The staff act on behalf of the railway (which acts as one entity for these purposes) and choose to sell you a ticket for someone else.
It's not strictly true that laws should be interpreted strictly. I'd be heppy to get in to the three different rules (which aren't actually rules) and two different systems for interpretation if you want to*.
I understand the law to mean that in strictness it is illegal to buy a ticket for someone else when the staff don't know you are buying for them, although in practice it is unlikely you will be prosecuted for buying a ticket that is valid for the person who uses it. The offences technically committed would be fare evasion* and numerous byelaw 21 and 22 offences, as well as byelaw 17/18 offences (which are arguably fare evasion).
If the staff know you are buying for someone else you are in the clear.
Authorised person means
i) a person acting in the course of his duties who:
(a) is an employee or agent of an Operator, or
(b) any other person authorised by an Operator
plus coppers. An operator could authorise people generally, as they do for the management of some station car parks. It would also be possible for the shop staff to be agents of the operator.
*I can elaborate on anything starred if anyone is as sad as me and wants to know more.
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Just noticed the tony_mac bit about tickets without fares on them being illegal on the railway. I thought that until someone else on here pointed out I hadn't read the footnotes on the SLD and that requirement had been repealed.