It's been said several times, for example when I raised the issue of staff on a company internal website advertising business tickets that they can no longer use, or people doing informal deals amongst friends and relatives, that this is TECHNICALLY a breach of the conditions BUT the rail industry is not concerned about this, however they would be if there was a huge market in train tickets on a site like Ebay.
You can go several ways on this.
You can point out the railway bye-laws which less than 1% of the population will be aware of and declare that they must never ever be broken or you render yourself liable to prosecution and a criminal record as well as being a Very Bad Person.
Alternatively you can say that the rail industry is a mighty scam anyway and whatever you can get back from them by any means, legal or otherwise, is fair dos.
Or you can say that there are practices which whilst not considered fraudulent by reasonable people in the sense that all journeys taken have been paid for by somebody, but are technically not allowed and you therefore take a measured view on the extent to which you participate and the likelihood of action. For example, buying an unused ticket from a mate at work is a whole different ball game from buying one from a stranger at a station barrier with RPIs watching although there is surely no difference in law. There's TECHNICALLY and then there's PRACTICALLY - take advice, some of which may be flawed, and choose where you stand, but remember that the law is a right mess and is biased towards the TOCs.
MSE is very good for highlighting a few basic ways of legally cutting costs, notably split ticketing, and gives a warning about dodgy ones such as short travelling whereas this forum is definitely geeky with a lot of the discussion based on obscure easements and suchlike - fun, but not what your average passenger is looking for.
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Sorry, I hadn't read Yorkie's post and what he says is quite reasonable.