Depends on how far you are into the deal. If you are still in attorney review and home inspection it's possible. If you are past all contingency deadlines then it will be a little harder without the possibility of consequences. If you have an attorney, talk to them and see what your options.

Technically, you are obligated to move forward with the purchase. If there are contingencies on the offer such as a home inspection, financing, etc and those items are not satisfied as stated within the offer, you would not be obligated to purchase. It would be best for you to discuss your concerns with your agent . (Hopefully a buyer's agent is representing you). You might also want to contact an attorney to give you guidence. Sometimes if it's early enough in a transaction and a buyer just comes clean with the seller on their change of heart, the seller may be in agreement to just part company. You may lose your earnest money or worst case scenario you could be sued for breach of contract. An attorney's advice might serve you best, especially if the agent that wrote your contract is representing the seller.