You do get a warning when selling any item much lower than its market price. I once did that with a white partyhat on accident. I missed a 0 and sold for much lower than market price, a warning popped up, I just clicked yes because I'm impatient, and then it instant sold of course. But I still got near the market price....lost about 20m only.
Edit: So that does lead me to believe this is a fake video. Not only because of the warning not popping up, but because the only way he would of only got 231 gold for it is if no one else was buying a purple partyhat for a higher offer.....which is highly unlikely.
For example, you want to sell bandos tassets quickly so you put in for 1 gp....it sells instantly and you get near market price still.