They should (and in my experience most do) sell you the cheapest ticket from your origin to your destination* which allows you to travel on that train (and return when you plan to, if applicable). (Unless you're travelling from a station where you could have bought a ticket, when of course you may only have the choice of an Anytime Single). Most people won't ask for a specific ticket type, just a single/return to X, and will be sold the cheapest suitable ticket.
I find ticket salespeople (both on and off train) will often delve a bit deeper, for instance asking about a return journey when people are asking for a single (usually pointing out that it's no more expensive for a return, or that it doesn't have to be a day return), or asking if the customer holds a Travelcard for London journeys (where a Boundary Zone ticket would be cheaper).
*Offering split tickets or tickets which start or finish beyond the station(s) required by the customer, which may be cheaper, is against guidelines so I doubt many guards would offer these.