Gazumping is when a seller accepts a higher offer from another buyer after already agreeing a sale, before contracts are exchanged. Because an agreed sale is not legally binding until exchange, the original buyer can lose the property despite their accepted offer.
In one line: Gazumping is when a seller ditches an agreed buyer for a higher offer before contracts are exchanged.
How gazumping works
In England and Wales a property sale becomes legally binding only at exchange of contracts. Until then either side can walk away, so a seller can accept a better offer at any point before exchange.
A buyer with an accepted offer of 250,000 GBP might have spent around 500 GBP on a survey and several hundred more on legal fees, then lose the home to a rival offering 260,000 GBP, with no automatic right to recover those costs.
Moving quickly to exchange, and using a lock-out agreement, can reduce the window in which gazumping is possible.
Gazumping vs gazundering
Gazumping favours the seller: a higher rival offer displaces the agreed buyer. Gazundering is the reverse, where a buyer lowers their offer just before exchange, pressuring the seller.
Both exploit the gap between offer acceptance and the legally binding exchange of contracts.
Primary source: GOV.UK: Buying a home