As itemframes are not protected (lockable) by any plugin atm they had to be disabled. Everyone could break them and steal the item.

If pistons and lava were enabled, I would have a nice self-powering infinite cobblestone generator.

Harry, cobblestone generators have nothing to do with this redstone contraption and honestly I find cobblestone generators are ok in my opinion its just the dangers of lava and the exploits of pistons and thats why they aren't allowed and wkas, minecarts are the only thing I can use without worrying about people exploiting my redstone contraption for personal gain. I can't rely on the honor system since we get alot of new players and as you know you can never truly trust someone you met online. Minecarts are a secure method I find since they won't be able to cheat the vending machine. If there was a redstone device that detected block ID's, then I would probably implement something like that, but there isn't so I am forced to resort to minecarts. Anyways, as of now I'm working on the vending machine trying to make a counter that counts up and down so in case people accidentally put in a coin they can get a refund however this is gonna take time as I need to make sure my contraption can't be exploited.

How about instead of having to build a full adder circuit for counting your minecarts, just simply have the carts line up on a series of detector tracks. If the player inputs two minecarts, then two detector tracks will go high, and then you can use a simple decoder to translate from detectors to dispensers to make them count off two items.

Refunds could be given from a bank of minecarts, instead of directly giving back the minecart they entered. This would prevent having minecarts possibly going two directions on any piece of track, and would allow declines to hold minecarts in place while being counted (since they wouldn't need to go back uphill).

Just some thoughts

Full adders are fairly large, especially for several bits, that's why I suggested the detector rails for multiple carts. If you're only doing up to 3 carts then a full adder would probably be fine, but going higher would likely be slimmer with the detectors.

I guess it depends how you handle purchasing, if you make your purchases all with one selection or you make several individual purchases. The latter would definitely require the ability to count up and down and the detector series would not work (well, anyways).