How do i check if a player has an item, and a required amount to give him something and get rid of the required amount.

So im using a gui, ive been trying to see how to check if a player has a SPECIFIC item and a required amount to give him something and get rid of the required amount. Without having itemstack...

Im creating like a shop, u go on a computer, u buy something. Simple kinda but i dont know how to do the first part.