Bava Batra 88 - Just Weights

A wholesaler must wipe his measures clean once in thirty days, and a householder, once in twelve months. This law applies to measures that are used for liquids which 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 in twelve months, and a householder - once in thirty days.

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