Can I sue the buyer for mail fraud if I ship FedEx to him originally?
As I wrote previously, anyone can sue anyone for anything. However, (1) mail fraud is a crime, not something for which a private lawsuit can be maintained; (2) there's no mail fraud if you didn't use the mail; and (3) nothing you've described constitutes fraud.
Is it illegal if I google him online and track him down at his workplace?
No, but what you do with such information may or may not be legal.