How do I make purchases for a backpack/inventory shop?

I'm trying to make a backpack/inventory shop similar to the one in Mining Simulator (when you click arrows, the camera moves and shows the next item, that's the best description I could think of so just check out the game if you don't know what I mean). I've gotten almost all the stuff down, the only thing I am absolutely baffled with is how to make purchases go through, and how to save the purchases...basically, everything under purchases. Could I have some help? If you need more info please ask.