A contract for real estate has to be signed by all parties (seller and buyer) to be legally enforceable. A verbal contract is not legally binding in the case of real estate.
I'm in agreement with Bill and Dan in that a verbal acceptance on an offer won't stand up to one that is in writing and signed by the parties involved. Your agent can keep a watch for it on the MLS in case it comes back on the market but it might be best to just move on.
Prudential Connecticut Realty
A verbal offer (or response) isn't worth the paper they aren't printed on. If you wanted to pursue this in court to try and get it enforced the judge would need some proof, which would only be your agents word. While this may be valid, so is the other offer that the sellers signed.
I'm not a lawyer and so my opinion may not count, but if you want to press it further you'll need to talk to an attorney. Sorry this didn't work out as you'd hoped.