And what about those of us to know EXACTLY what we want? And would never ask for anything that wasn't allowed or was not available (except for perhaps an Advance might have disappeared in the time since checking online and asking at the booking office window.)....
If it is apparent that the purchaser knows exactly what they want, and it is valid and available, then a clerk need not ask any questions. If I ask questions of a passenger, it's not because I doubt anyone's knowledge, it's because I want the passenger to get the right ticket first time round.
....I know that for some passengers it might help if they are questioned. I personally though always know exactly what ticket I require (he that for travel then, or in the future, or from another station, or a ranger or something else.....) and any additional questioning - moreover if the person selling it (Merseyrail being my local station hink wink) doesn't understand the ticketing system as much as I do - just delay my transaction or causes hassle for me having to have a pointless conversation....
As with any industry, some people aren't as good as they should be, that's life I'm afraid, but for the rest of us please look at it from our perspective. How are you different from anyone else? What would tell us that you know what you are talking about?
....To put it another way, can a full service booking office refuse to sell somthing they are asked for it is available/possible.
There are situations were a sale should be refused, so the answer to your question is, yes they can refuse, but I would add that, provided the ticket you want is both valid and available, there is usually no reason to do so.