When a seller signs a contract to sell their house, they have to do just that, sell it. And, if it says they have to replace the septic tank before they close, they have to. As far as closing goes, what is the closing date in your contract? Each state has different customs as far as how far past that date you can go before you are truly in violation. The contract never specifies what the seller plans to do after closing, other than vacate the premises by a certain date, if they are not out by the closing date. Again, sometimes it is customary to give the seller 5 days to move out. What does your agent say? Are you represented by a lawyer?