The seller always has the option of accepting a higher offer or a Buy It Now from another buyer even after you've sent them your offer. It has always been this way, AFAIK.
Bidding Frustration
This past weekend I placed a bid on an integrated amp and the seller countered. My understanding is the counter was to be open to acceptance or rejection for 48 hours. I gave it some thought and about half a day after the counter was prepared to accept it but it was gone as was the listing. I e mailed audio gin and the answer I received was yes a counter is good for 48 hours unless the seller pulls the listing. Logic indicates that while the counter was pending the seller received another offer he accepted and pulled the ad. My opinion is not cool. Thoughts?