Let's put this one in a chronicle order:
Your first offer to sell was $170k. Buyer countered with 150k+3%. At that moment your 170k offer was gone and only thing left was her counteroffer. When she offered 160k+3%, the original offer was gone. If you checked the offer and if you used official forms, there is a time limit on the offer and the original 150k+3% should have expired or at least revoked by the buyer when she made the 160k+3% offer.
At this moment, there is no 150k+3% offer that you can accept. When you signed the wrong offer, you basically made another counteroffer to her 160k+3% and this will cancel 160k+3% offer. As such, you do not have a contract and only have your couneroffer of 150k+3%, which she can accept or reject. However, should they notice this and hold you liable for the contract, you would have a mutual mistake defense anyway.
Of course neither the buyer nor her agent understands this and that is why they left the offer on the table but made another offer of 160k+4%. You can reject or accept this offer.
Now you can understand why you have no contract at this point.
