I don't fully agree with this.
The contract is out there, wide open to everybody, any potential buyer SHOULD know he might not be the only one interested, basically everybody has an EQUAL opportunity to calculate his chance, then take actions. Now buyer A chooses to make a reasonable offer, buyer B chooses to begin with a low ball, to me, round 1 over, buyer A wins, should proceed. But you are saying the broker now steps in, confirming something buyer B was betting/hoping would not happen? That is similar to letting buyer B know he miscalculated, i feel it is unfair to buyer A.