Legal Question in Business Law in California

I would say yes, as a general matter it's legal, but the seller would have to be careful not to violate any restrictions re packaging and shipment.

Mr. Whipple is wrong. It is biohazardous toxic waste. Unless you comply with all hazmat collection, containment, transportation and disposal rules for feces, you can't even collect it in commercial quantities, much less move and sell it. Though, I guess, if you could somehow comply with all the rules and laws and still sell it at a profit, maybe you could. Not that that is even remotely possible in real life.