Wondrous item, uncommon
Once per day, when two or more willing creatures have stated the terms of a bargain aloud in your hearing — goods, money, service, or forbearance — you can knock the hammer once upon a hard surface (an action). The bargain is struck, and the creatures know it.
For 30 days, a creature that knowingly breaks a term of that bargain gets nothing by it: money it received under the bargain is spent, mislaid, or found short; goods it received do not serve it, and any magic item so received is inert in its hands; work it was owed is done badly. Every creature that heard the knock knows that a term has been broken and by whom, however far off it is and whether or not it wishes to know.
The knock binds no one who was not a party and heard the terms. It binds nothing that was not said aloud.
The head does not fall upon a bargain in which a person is the thing sold. The hand feels the weight go out of it, and the crier goes on to the next lot.
Source: Provenance — The Quist Sale