Welcome! Thank you for your question.
Essentially he is claiming that you breached your contract
to supply him the guitar as described in your conversations and with photos. So long as you supplied him a guitar as you described and did not represent or hid anything that was material then you would not be in breach of contract
With all that said, you do have two issues: First, since the contract is not fully in writing most of the agreement will be based on oral testimony
with you saying one thing and him saying another. It will be up to the court to sort out who to believe.
The second issue is that it is possible that the court in Connecticut would take personal jurisdiction over you since you communicated over the phone, by email and shipped the product to the buyer in Connecticut. If the court takes jurisdiction over you then you will have to defend the lawsuit in Connecticut. This would be very time consuming and expensive from CA. The jurisdiction is obtained under the long arm personal jurisdiction statute. If you are sued then you need to hire an attorney in Connecticut to challenge the suit based on lack of jurisdiction. If you appear yourself then you will likely waive any jurisdiction objection.
I cannot provide you with legal advise. I have provided you with information about the law related to your question. My answer, and any information that you find online, should not take the place of having a consultation with a lawyer in your area to advise you regarding your specific issues.
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