Gazumping is when a seller accepts a higher offer after already agreeing a sale with you, but before contracts are exchanged. Nothing is legally binding until exchange, so it is entirely lawful and cannot be prevented outright. What helps is speed and goodwill: ask the seller to take the property off the market once your offer is accepted, instruct a solicitor the same week, and return your paperwork promptly. The shorter the run-up to exchange, the smaller the window for someone else to appear.