Hmmm - I don't know who the person is and which thread this was but from my understanding the sales thread is binding. I don't see how he could have mistakenly posted the domain for sale at $100 instead of $400, I doubt it was a "mistake" at all. Either way, even in the unlikely event that it was an accident/mistake it's still his responsibility to honour the agreement....it was his "mistake" so the onus is on him to accept the consequences of the mistake and deal with it.....
Prob best to let everyone know who he is so other members don't end up in a similar situation with him.