Penalty for canceling sale of home after fully executed contracts

In some states there is a period of time within which you can back out (often 3 days).  After that, assuming you had no valid reason to back out (e.g. the other side had breached a material term, or defrauded you into entering into the contract, etc.) you would be in breach of contract.  The buyer would likely sue you for specific performance (i.e. to have the Court compel you to perform the contract) and/or any damages he/she/it sustained from the breach.  The realtor would also sue you for the commissions he/she/it lost from the sale.