I accepted an offer to buy a house in FL. They live in Indiana and I live in MA. If they refuse to sign the closing papers can I sue them?

If you accepted an offer to sell in the exact terms of the offer, then you have a contract.  The nature of a contract is to bind the parties to their written word.  (May I also assume you paid some earnest money?) If, after entering into a valid contract, the sellers backed out, you probably have a valid reason to sue.  However, it might be a mistake to do so.  You will probably have to file suit in Florida (or perhaps in Indiana), so geography alone will make the prospects very expensive for you.  Unless the contract specifies that you are entitled to attorney fees and court costs, this could be enormously expensive.  The most likely remedy would be a court order for the sellers to sell.  Unless you can prove some kind of financial loss resulting from their backing out of the transaction, you cannot recover damages.  A more practical response, in my opinion, is to write the experience off as a bad one, and get on with life.  I would certainly ask for the earnest money back though - and I could not imagine difficulty in getting those funds back.