Bava Batra 48 - Is a Forced Action Valid?

If they subjected him to distress until he finally relented and sold to them what they demanded, his sale is a valid sale, and he cannot retract.

What is the reason?

* Is it because any sale is forced, since the seller needs money? - No, it could be different when it is against his will.
* Is it because a recalcitrant husband who is forced to give a Get is indeed divorced? - This is not a valid proof, because it is a mitzvah to heed the directive of the Sages.

Rather, it is derived from the following ruling: if they subjected a woman to distress until she agreed to be married, the marriage is valid. This proof stands. However, the Sages later annulled such a marriage: he behaved improperly, therefore, they behaved improperly with him.