Was your offer accepted?
I don't think that's a formal acceptance of your offer. He uses the words "would be willing to accept" as opposed to "I accept". He also refers to an email to follow which presumably has terms and conditions attached. So the contract at that stage is conditional on you being prepared to accept those terms and conditions. I think you can effectively defend their demand for payment on that basis. I hope that helps. Please leave a positive rating so that I am credited for my time.
Tried to call via system but no answer yet. However the further emails are indicative of a contract: you have an offer and an acceptance. Technically they could sue you if you don't pay. However whether they would in practice is another issue. They may just keep the domain in stock and seek to the next interested buyer. I can't guarantee that obviously but many commercial organisations would find it a waste of time to sue for that amount of money. However the answer to your question is that there is a binding contract based on what you have now told me.