“I had an affair with a married man, the relationship went bad. I went to one of those online cheating web sites and posted his information, not his home address or phone number, but vehicles, location and name, age, etc. Are these web sites legal? Can he sue me for Libel Defamation of character?”
He can always sue you; whether you will lose or not will depend largely on whether everything you said was the truth.
However, he could conceivably sue you for other things, such as intentional infliction of emotional distress, and various other personal injury (tort) actions.