The word gazump is used mainly in Britain
and, to some extent, Australia
. It is used in the context of trade and means to reject a previously accepted offer to buy at the last minute in favour of a better offer from another party.
It is derived from the Yiddish word gezumph: "to cheat or swindle"
The word first came to use in the 1920's, conveying the general sense of overcharging for something, but it is now used solely in the context of house purchase.