Hmm I'm still not convinced that the newsagent is necessarily doing wrong.
If he bought the stamps from the Royal Mail under contract, then yes, at least potentially. Maybe the legality of such a contract needs testing though. Resale Price Maintenance is generally illegal in the UK and has been for 40+ years. I don't see why the Royal Mail should be exempt from the law.
If he bought them from a Post Office or another retailer for cash, I can't see that he's bound by any Royal Mail T&C - he's not contracted to them in any way. I think he could argue that he's charging his customers a service fee for obtaining them stamps at cost.