I had to Google it after a homeowner referenced it when I was meeting with them this week.  It seems it is more common in England and Wales than it is here -- thankfully!

Per Dictionary.com...

gazump - to cheat (a house buyer) by raising the price, at the time a contract is to be signed, over the amount originally agreed upon.

Some sellers in this market might price their home extraordinarily competitively to aim towards having multiple (above asking price?) offers to consider -- but they don't typically gazump.

Homeowner concludes that her home is worth $300K.  She lists it for $295K and is delighted to receive offers for $295K, $300K, $302K and $304K.  She sells the house for $304K.  Not gazumping.

Homeowner concludes that his home is worth $300K.  He lists it for $280K, receives an offer of $290K, tells the buyer he is going to accept it, and then changes the price to $305K after the buyer has signed the contract, and demands that the buyer initial and approve the change.  Definitely gazumping.