Actually you had this on your sign:
wanteddead411
16
B:2000 S 1
Iron blocks
As the ":" acts as the seperator between the Buy and Sell side (if you put both on a sign) it saw the "2000 S 1" as the number for what you bought for.
So this way you actually bought them for 20001.
The correct way should have been:
wanteddead411
16
B 2000 : S 1
Iron blocks
A painfull mistake. But a good thing the player was kind enough to give the money back
Topic closed as this was solved