Clicking the button will activate a setblock to the 2 command blocks BUT if the players have less than 500 resources or 100 gold than they could just click the button to get a bunch of the item and their score or money would go negative.
Therefore if your not use /testfor @p[score_scorenamehere_min=500] and let the signal it outputs go into another command block that takes away that amount of score.
That way the player can't have infinite amounts of the item that's being sold.
