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

I gave an offer to an amp recently and seller refused the offer through message. I moved on, and after 1 and half day seller accepts my offer without taking my approval and since I did not purchase he gave me a negative feedback, and audiogon charges for backing out. Am I wrong here ? I would have definitely bought the amp if he had accepted my offer at first place or gave me a counter offer which I accept.