Chances are you can't sue. The standard real estate contract used by realtors in Pennsylvania typically states that if the seller walks away prior to closing, the buyer can keep the deposit money. This is actually substantial protection for the seller, because the traditional rule was that in real estate when the buyer walks away, the seller could sue for specific performance — i.e., they could FORCE you to buy the house.
Review the contract carefully — I'm willing to bet that the language I described is in there. If it is not in there, then yes you can sue for your money back (but then the seller can also sue for specific performance.