Bava Batra 88 – Just Weights

A wholesaler must wipe his measures clean once every thirty days and a householder once every twelve months. This law applies to measures used for liquids that leave a residue, such as wine or oil. Since the residue tends to settle in the measure and congeal, the measure must be cleaned periodically. Since a wholesaler sells frequently, his measure tends to gather residue more quickly.

Rabban Shimon ben Gamliel says that the opposite is true. Since the wholesaler uses the measure more frequently, the residue that gathers in it does not congeal between uses. Therefore, the wholesaler must wipe his measures once every twelve months and a householder – once every thirty days.

The phrase “A perfect and just weight you shall have...” also teaches that the seller must act justly and give the buyer a little extra.

Art: An Old Man Weighing Gold by Quiringh Gerritsz van Brekelenkam