it is not hard to accidentally buy something from a vendor with your marks that you didn't mean to buy. It would be nice if they made you press the purchase button longer (like when dismantling exotic gear) or allow refunds if you haven't left the tower yet. it's kind of a big deal when you accidentally buy the wrong thing with your marks because of how much time has been wasted for doing so. As if I don't spend enough time playing the game already