You're talking about a time pre-capitalism. "Incentives" don't map to a stable cross-community market. We should talk about "reasons," not "incentives." A great reason not to try to deceive about weights is it would make nobody wants to trade with you when it gets out that you're dishonest. It may have also been considered deeply unethical in their society to do so, there doesn't need to be any enforcement mechanism beyond that for it to manifest across the entire society in an archaeological record.