I don't think using multisig addresses for openbazaar donations is going to help anything. Anyone that facaliates the sale of illegal drugs (or any other illegal good for that matter) is risking apprehension by law enforcement as well as a lengthy jail/prison sentence
Even if you ignore the incentivization from asset forfeiture, there are real limits to how an organization can fund its operations without sufficient funding. Right now a large amount of funding is done through fines and theft(asset forfeiture). If one uses mutisig addresses than these apprehensions will result in less theft occurring therefore starving the state of these needed funds.
Where will law enforcement make up this loss in revenue?