BEFORE YOU RUN THIS COMMAND, DO 

THE FOLLOWING:

1. Copy the command and paste it into Notepad

2. Press Ctrl+F

3. Search for PRICE ; change all the PRICE 's to the amount of XP levels you want the item to be sold for

4. Search for ITEMNAME ; change ITEMNAME to the name of the Item to be shown on the sign

5. Search for ITEMNAME2 ; change ITEMNAME2 to the name of the Item to be used in /give

Once you're done; copy the new command and run it into a command block! Tada!

You need to be kinda close to the sign to use it; this is to stop people from making others buy items.