They have the same duty as any executor, that is to act reasonably and in good faith in administering the estate. They are not bound to sell assets of the estate in any particular way. It's a matter for them how best to discharge their duty . They should get a reasonable market price, but how they do that is for them to decide. It may be better to sell without using an agent or even to do so without advertising, if they have got proper and competitive bids from genuine buyers without. They couldn't be criticised if they get a full cash offer at the current market rate from a bidder whom they know is certain to complete quickly and without problems, rather than waste time and expense on doubtful offers.
If the property is in Scotland they wouldn't usualy use agents anyway; there solicitors act as estate agents, the system being different from ours in England and Wales.