As I mentioned earlier, that person was looking to buy Imclone stock at that particular time, anyway. If Martha hadn't been selling her stock, the buyer would have just bought it from somebody else who was trying to sell it at the time. Whether there had been insider trading or not, the person who bought the Imclone stock would have bought it anyway.
Yes, if insider trading were proved, the purchaser has recourse.
Even if they do, there's no reason for them to get recourse. They incurred no damages.
Doesn't matter. IF (big if) Martha's trade was illegal, the trade gets busted. She gets her stock back, and a lot of legal aggrivation, the "lucky" buyer gets his money back.
Markets are to be as transparent as possible. It doesn't matter, but the buyer may well not have made his purchase given the knowledge Martha had. Actually, Waskal (the Imclone guy, right name?) is a much better example, since her knowledge is unclear.