It seems unethical because it is. Making what appears to be a genuine offer to sell at a given price then saying "haha, sorry, just wasting your time to help me

It seems unethical because it is. Making what appears to be a genuine offer to sell at a given price then saying "haha, sorry, just wasting your time to help me but without paying you for it, I'm not selling" is unequivocally a dick move.