if i post that i'm selling something, and someone replies "bin $xx fg". Does JSP consider this a transaction? Now this person is ghosting.. is this normal behavior? Am I allowed to just post "bin" on peoples posts and then not follow through? thanks.

Its considered bad mannered...
Usually good traders will buy the item if they say BIN

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.

Very nice

very nice getting 2 posts.