Here we go again.
I have quoted "in black and white" a rule that contradicts with that rule. In consumer law, if consumers are given contradictory laws, I believe that the one in favour of the consumer prevails.
It would also likely be ruled an unfair term, as nowhere does it state that a Railcard must be purchased in advance of travel if buying a ticket in advance.
It is likely that the statement in Rule 5 is intended only to apply for journeys for immediate travel where one passenger asks for one ticket for immediate travel where the seller believes that person is intending to use that ticket and has no other opportunity to obtain a railcard before actually travelling. In such a circumstance it is reasonable to insist on a railcard being seen. In many other circumstances it would be unreasonable, and I believe it would likely to be unlawful and perhaps even discriminatory. (e.g. say I am buying a ticket for next week for an elderly/disabled passenger. I AM allowed to buy it for them and I think I'd have good grounds to get you into trouble if you refused to sell me it, and believe me I would take it as far as I needed to take it.)