FormerlyHemlock
Hero
Yup. Merchants would use them all the time. It would be especially helpful with verifying documents. "To the best of your knowledge, are all the representations you have made in this contract as to the validity and quality of your wares correct and complete?" or even more simply "Are you withholding any information you think I would want to know?"... Evasive answers also don't have to be tolerated. You can limit the answers to "Yes" and "No", or rephrase your questions until "Yes" and "No" are valid answers. Failure to answer questions unambiguously could be considered presumptions of guilt.
"Yes" and "No" are actually more ambiguous and easier to abuse than full-sentence answers. I'd go the opposite way: I would not trust any statement made under a Zone of Truth which was not a full, declarative sentence.
"Are all of your wares of the quantity and quality stipulated in the contract you signed with my father one year ago?"
"Yes, Domenico, all of my wares are, to the best of my knowledge, of the quality and quantity stipulated in the contract I signed with your father one year ago" is harder to fake than a monosyllabic "Yes."