seller backs out of agreed upon transaction

[StJNOPN]

I am located in TX and the seller was located in Ohio. He was responding to an online classifieds ad. We had agreed (through e-mail and phone conversations) to the terms of a transaction. The next step was that the seller was going to ship the merchandise to me and then I would pay for it (We were using an online escrow service to facilitate this.). Then the seller contacted me saying that he sold the merchandise to a local buyer. What legal recourse can I take in this matter. The merchandise goes for ~$600. Thanks in advance for all of your help.