Hey everyone! Today I'm gonna be teaching you how to make a shop in vanilla Minecraft, using experience for currency. Feel free to leave a diamond!
NOTE: XP= Any number you want
B= the ID of any item you want
A= Any number you want

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Step one: Gather the materials
-2 Redstone Repeaters
-2 Solid blocks of your choice
-1 Button
-3 Command Blocks (You can obtain command blocks by type /give @p command_block)
-1 Sign (Optional)

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Step two: Enter the commands.
-Place one command block and type 

give @p[lm=XP] B A

-Place a redstone repeater going AWAY from the command block in any direction you want, then place a command block so it gets the signal from the repeater and type

xp -64L @p[lm=XP]

-Place a command block on top of the first one by Shift clicking the top of the command block and type

tell @p[l=63] Sorry @p you don't have enough levels for these items.

-Place a 2 block tall column of any solid block you want. Then place a button on the bottom block. Type in chat

/xp XPL

-Press the button once, it will give you the items and take the levels from you. Press it again, if you don't have enough levels it will say "Sorry (Closest players username here) you don't have enough money for these items."

Let me know if you have any problems or questions!