Under the most basic elements of contract law, a rejection of an offer ends the matter right there.
If the terms of the Audiogon service clearly and unambiguously provide for a contrary result and there is proof that the buyer assented to the terms by a specific opt-in to the terms (and not just by using the site after the rule went into effect), then maybe the would-be seller has a leg to stand on.
If the terms of the Audiogon service clearly and unambiguously provide for a contrary result and there is proof that the buyer assented to the terms by a specific opt-in to the terms (and not just by using the site after the rule went into effect), then maybe the would-be seller has a leg to stand on.