Blago wasn't convicted of selling Obama's seat, they got him for wire fraud, and attempted extortion. It is speculated the Feds quietly pulled the wire down because Jug Ears was said to be involved.
That was my theory at the time it happened. What was the big hurry to arrest Blagojevich? Obama wasn't going to be sworn in for another month or so. They might have caught Blago in the act if they had let it play out..