Seller says they meant to counter but executed our offer!?!

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Hello everybody, 

We were able to find a great property listed through the MLS and being offered through an estate. Rehab costs along with our offer appear like it will put us in a great position to execute the BRRRR strategy as the ARV looks promising. We offered significantly less than the listed price and the seller executed and accepted the standard Colorado Contract to Buy and Sell! After several hours of further analysis and initial excitement in the fact that it appeared we got a great deal, we were hit with a curveball.

The listing agent informed our realtor that the seller actually intended to counter the offer, not accept it.  The Colorado contract, as I'm sure is similar to most, has clear delineation in the contract where the seller should sign if accepting versus if they plan to counter or reject.  As far as we're concerned, we have an executed contract with acceptance of our offer but have already been informed it was a mistake by the seller.

Has anyone run into this before?

Does the seller have anything to stand on to back out?

We're ready for a battle but looking for any feedback...Thanks!