National legislation offers governments limited possibilities to regulate sustainability aspects of materials and products entering national markets. It is often difficult to enforce requirements, because the adherence to sustainability criteria cannot be judged based on the product itself. It requires proof of conditions at the country of origin. These conditions are difficult to verify and vulnerable to fraud. The timber example (Section 3.2.2) requires importers to provide declarations, using ‘due care’ that imported wood has not been harvested illegally. For a customs officer, ‘due care’ is rather difficult to control, without the presence of standardized and trustworthy mechanisms to provide evidence on sources and production conditions.