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

Jmcgrogan2, i hope you realize that i was praising your statement and was referring to the seller.

Something i just realized is how vindictive the seller was to post feedback on a non-transaction, assuming the facts are correct.

I have been very active on this site and you can see that from my feedback. I must say i don't understand the negative feedback given to the poster. The seller sent an mail saying he does not accept the offer. Sure, not on the Agon strict and painfully slow system, but still gave his intention in plain writing.

Goodness, simply show this email to Agon and remove the negative feedback. Common sense must play a role here.

Yes, we all must use the formal system to avoid this sort of thing, but here we have an obvious refusal of an offer, in writing, and common sense must prevail.

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.