Make sure you understand what your remedies are in this situation. If your contract says that you agreed to mediate any disputes, then you need to follow that procedure before considering to sue the seller.
Note that your agent, or you, need to make it very clear to the seller that you have a binding contract and that you expect them to follow through with their commitment. If that doesn't get you any where, then follow up that discussion to let them know that you're going to pursue all legal remedies at your disposal, and that those remedies will likely tie up the property and keep it from being sold to anyone until the dispute is resolved.
Often times the sellers try to back out of a contract because they get a better offer, but if they know that you are serious, AND that you intend to tie up the property, they will quickly realize that they can't even sell it to the higher offer because you're going to put a "cloud" on the title with your dispute.
To help your case, make sure that if you have an accepted offer, that it's in writing with everyone's signatures on it. In most states, a verbal agreement is not defensible in court. If you have the signed agreement, then take it to a title or escrow company to be receipted. Often times the seller will stipulate a preferred escrow or title company, use that specific one.
In addition, make sure a copy of the signed contract is delivered to both the listing agent and the listing agent's broker, and inform them they they too are expected to follow through with the commitment that their client made to you.
Keep us updated on your results!