In general, sarcasm, exaggeration, biting criticism, parody, etc., that a reasonable person can be expected to not take as literal truth, are not considered by the courts to be defamation.
However, referring to a member as "deranged" is a violation of Audiogon’s terms of service. Note that if you point to the lower right corner of any post a "report this" link will appear. If you click on it you will see that one of the reportable categories that is listed is "abusive towards another member."
Just FYI. Regards,
-- Al
(Licensed but non-practicing attorney-at-law)