That's where the competition comes in. If you have more than one regulatory agency, they have a huge incentive to catch their competition out at dirty dealings. A company would outlast their reputation only by days. You bribe a government monopoly agency, some poor dupe loses his job, but the agency keeps right on going. Rinse and repeat.

Wouldn't even need laws demanding the oversight. Like with Underwriters Labs (a private company) you get a stamp or something on your product showing you've been through the testing or other oversight system. Customers would look for that. Wouldn't you? A product with stamps from more than one consumer protection company would be more marketable. You could have specialization. Maybe some companies certify for organic. Others for resource use, or health effects. The more, the merrier. And as with any private endeavor, the ones who do the best job make the most money. Make market forces work FOR you. You're a sailor. You know how to deal with wind to make it work for you rather than against you.

I should think this out completely and write it up someday. Maybe in the form of a fiction story to make the point in an entertaining manner...