I think they work especially well when money is involved. If you make some money from time to time by subletting your property or sharing a ride you have even m

I think they work especially well when money is involved. If you make some money from time to time by subletting your property or sharing a ride you have even more incentive to maintain good reputation. This is how I imagine small early human communities worked: based on reputation. The thing is that our brains don't scale and they can't maintain reputation model for all people whom we exchange with in a large city. Services like airbnb and uber externalize the reputation model to a large scalable distributed system. As far as spammers and other abusers are concerned tech community has a long and largely successful history of dealing with them (blocklisting, statistical techniques like bayesian spam filtering, machine learning algorithms etc). Gmail is a very good example of effectiveness of spam-fighting techniques.