A couple of basic thoughts that are misunderstood by people in a Real Estate Transaction. First, an offer is never complete until there is an acceptance and that acceptance has been communicated to the other party.
Sellers make a counter and give three days for a reply. One day later they get a better offer. They can accept the second offer without giving any notice to the first offeree.
If the buyer accepts and signs the counter offer and gives the acceptance to their agent, the seller can still sell to someone else until acceptance has been communicated to them. Some agent friend of mine just had that happen yesterday. The buyer wanted to think it over. They will have a lot of time to think it over now, as that property is gone. Yes, they can become a backup offer, but that could be like being in a standby line at Southwest Airlines in San Francisco on a Friday night.