Technically we don't have any rules against backing out of BINs or sellers/buyers not honoring BINs but as Pazar said that is considered bad mannered or "BM" for short,
Users have the right to retract any trades or sell items to whoever s/he wishes at any given time,
however we do monitor users for intentional disruptive behavior such as if a user is posting BIN in multiple trade topics but never go through with the trade, fake bidding is considered spam.