What is Gazumping?

Legal Definition
Gazumping occurs when a seller (especially of property) accepts an oral offer (a promise to purchase) on the property from one potential buyer, but then accepts a higher offer from someone else. It can also refer to the seller raising the asking price or asking for more money at the last minute, after previously orally agreeing to a lower one. In either case, the original buyer is left in a bad situation, and either has to offer a higher price or lose the purchase. The term gazumping is most commonly used in the UK, Ireland and Australia, although similar practices can be found in some other jurisdictions.
-- Wikipedia