Am i obliged to buy when seller refused at first..

you're out $19 and some trivial feedback but you dodged a bullet by not having to do business with a seller like that.

From what I see here from the OP, is that he received a refusal to his offer and didn't counter offer. The negotiations ended there. Any return to the "table of negotiation" would have been either a "if you still want to offer X, I accept?" And a required response of acceptance or refusal would have re-opened the negotiation. But the sellers original NO was the end for both parties unless the buyer responded. In any world "NO" is a complete sentence.
Audiogon needs to research and correct this, but in the meantime, we all can learn from this one. When your offer is refused, respond with a closing comment that your offer is off the table or the option of a "let me know if you change your mind".
Just my opinion of course.

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.

02-01-15: Lse
Under the most basic elements of contract law, a rejection of an offer ends the matter right there.

While this is true, the seller did NOT decline the offer through official Audiogon channels. The buyer made an official offer through the Audiogon system. The seller PM'd the buyer rejecting his offer, off the record. He did not officially decline the offer through the Audiogon system though. That is how he was able to accept the offer. It's like he tried to strong arm a better deal, when the buyer balked, he accepted his original offer.

I assume in the legal world there is a big difference between working with official documents on the record, and working off the record, is there not?

Cheers,
John

Not sure about that john. A contract is a contract is a contract. Oral, written or otherwise. Now proving it is another matter. Audiogon Should get right on top of this, because this is clearly a "bug" in their system that undermines the formal more profitable for them system that they have tried to establish. One of the motivations to stay within that system is that it is supposed to provide at least some protectton to buyers and sellers from someone who is negotiating in bad faith. The ability to refuse an offer rather than decline it, while leaving the buyer on the hook will drive buyers away. Without buyers there will be no sales. Without sales no sales charge and eventually no sellers.