I am very sorry to learn of this situation. You have a very complex issue due to the vague nature of the text messages between the two of you.
However, what you can do is sue both the individual that was sitting your dog, and the person that currently has your dog, in small claims court for the tort of "conversion" (it is the civil equivalent of theft).
In that small claims case you can argue that your initial text had not been an offer of a gift to the other person, and no reasonable person would have interpreted it that way, rather it was a post to see if there was any interest in purchasing the animal.
As she stated she had no interest in purchasing, there was no need for any further communication in that regard.
However, be very aware - your case is by no means guaranteed, and the nature of the texts as you have posted them here could possibly be interpreted as you offering to give away the animal (your personal believe that the dog was worth a lot of money notwithstanding).
But, that should at least give you an avenue to pursue to get your dog back.
Unfortunately, this is entirely a civil matter, not a criminal one, law enforcement is not going to be able to assist you until you actually have a civil judgment.