Here is how the setup should be.
The sign says "Diamond Block $1000". Make sure to put a button on the stone. In front of the Stone put a repeater. Then put a Command Block(1). Then you will put a repeater in front, set it to 3. After that, put a Command Block(2) and another repeater in front(Don't Change). Put Command Block(3) in front and one more repeater in the front. Finally put a Command Block(4).
Block Name Command:
Command Block(1) - /Tell @p[lm=1000] Purchase Successful!
Command Block(2) - /give @p[lm=1000] diamond_block 1
Command Block(3) - /xp -1000L @p[lm=1000]
Command Block(4) - /tell @p[l=999] Not Enough!
Let me know if that help you. :)