Interesting - so are you saying the ticket office has no responsibility to check validity of tickets before selling them?
The ticket office has joint responsibility with the passenger to ensure that the correct ticket has been purchased. However they can't, as you suggested, refuse to sell a ticket unless they have reason to believe that it will be used fraudulently. So, in the case of a child ticket for immediate travel they could legitimately ask for proof of age before selling the ticket. If the customer was unable to provide such proof but was insistent on buying the ticket then most offices would sell it but endorse it to the effect that the customer had been advised of the age restriction.
But for a railcard-discounted ticket for future travel they are under no obligation to make any checks other than ensuring that the customer knows that the ticket is only valid with a railcard.
Presenting a railcard while purchasing the ticket likely sufficiently satisfied the clerk that you were aware of this, so I can fully understand why they didn't examine the card for validity.
