You know back in the days you can also go to farmer Joe and buy a chicken and pay no middle man. Jumping from that fact to the conclusion that food supply chain has not much value is, naive, to say the least.
If you cannot get everyone to review for all purchases, then the next best thing is to do random sampling by randomly choose buyers and purchases to get reviewed, and possibly pay them so that selected reviews are collected. I have fond memory of winning the moderation lottery at Slashdot [1]. The current system of free for all will only amplify the voice of the abusers.