Do you really think he began discussing the "sale" of the seat only after the Nov. 4th election?
No, but I would imagine that the prosecutor wanted to wait until the seat was actually guaranteed, so that the conversations were based in reality rather than just a possibility. He was probably waiting to see who would come around looking to buy the seat, and what other information he could gather on the Chicago political machine.
I think Fitzgerald was absolutely correct in getting as much of the discussion on tape as possible, and making sure his investigation was complete. He is well-known to be a very thorough prosecutor. He likes to have so much evidence that the conclusion is never in doubt.
It is unfortunate that the information was not out in time for the election, but Fitzgerald is too professional to let the election dictate the results of his case.