Once an offer has been formally accepted and the vendors and purchasers agree on the terms of the sale, contracts are signed by both parties, a deposit is paid and then contracts are exchanged. The contract is now legally binding. There is then a 5 day cooling off period. After this time the sale/offer cannot be withdrawn.
There is no cooling off period if you buy a property/exchange contracts on the day of an auction or if you have a 66w certificate.