Implicitly an offer to sell at a stated price and containing "obo", such as $1000 obo, means the first person to accept at $1000 has formed a contract by giving the required acceptance. The obo portion means that if an acceptance is not received on the sellers terms, then the seller will take the best offer short of the stated offer. Any proposed offer which differs from the sellers terms becomes a counter offer. For example, if a potential buyer emailed and said, "I accept your offer if you include shipping" or "I accept subject to my spouse's approval," are all offers which become counter offers and put the power of acceptance back to the seller. If a buyer did not vary the terms of the offer, then the buyer has the power of acceptance subject to a prior sale. If the buyer is the first to respond and has truly accepted with no counteroffer, then one should sell to the first to accept the offer. At the time of acceptance a contract has been formed.
I recently had a seller make an offer, then when I accepted, he refused to sell even though I was the first to accept on the same terms as the offer. Instead of providing any reason why, he simply refused to acknowledge me with any further emails. After numerous emails over 4 days and no response, I left him negative feedback. Of course, he retaliated by leaving me negative feedback. This was a real disppointment as I feel Audiogon is a great forum. I have bought several items without a hitch as well as received great advice. To conclude, I got the impression that the seller felt he did nothing wrong and that he was right to just ignore me in hopes I would go away because he had second thoughts on selling the item.