I had this happen on the same item twice. I accepted a best offer from a 0 feedback buyer. A day later they sent a message asking ridiculous questions. I told them they could cancel if they want. They never responded and never paid. I cancelled it as soon as I was allowed and relisted it. Almost immediately another 0 feedback buyer sends an offer and I accept. I wait a day, then decide to check them out. They had  a vacant block as an address and some scientific term instead of a real name. I strongly suspect they are a rival seller with extremely poor feedback using a guest account to buy my item to keep it off the market while they attempt to sell theirs. I cancelled it as soon as I was able. I have since changed it to buy it now, no offers and immediate payment. They started watching it again as soon as I relisted it.

I'm onto you, rival seller.