'/testfor' a number more than 0

A good way to do this is to have it when you press the button, for something costing 3 'points', have a command block with the command /testfor @p[score_TEST_min=3] This will make it so the minimum score you can have is 3. So, if your score is smaller than 3, it can't take away points.

Remember, it can be any number, just substitute the number 3 for the number of points something costs. Lead a redstone comparator out of that command block, and to the command blocks that take the 3 points away from you, and give you the item(s). (I'm assuming you already have the commands for these, if you don't, leave a comment, and I will add them.)