The seller is free to withdraw the counter-offer any time prior to your acceptance of it. The communication method for acceptance is usually described in the contract. If your acceptance was communicated to the seller in the required method before the seller withdrawing the offer, the seller should honor the contract with you and not entertain other offers.  If the seller refuses to honor your contract, the problem becomes whether you try to enforce your contract or not, which requires legal advice and expenses. For that, you have to consult an attorney.  Although you could probably technically enforce the contract, you have to reach a decision on whether it makes sense to spend the time and money to do so, or weather it makes more sense to accept the seller is unethical and just move on to buy something else.