The thing that makes it amazing is nobody is violating the spirit of the free exchange. If people put out loads of junk it would break down. I mean in most cities if you just set up an area where people could leave things to give away, the fucking redneck scum would show up with pickup trucks and dump their broken washing machine. It would instantly be overwhelmed with garbage and nobody would ever go to take things away, just to dump trash. In SF people don't put out much junk for the most part.
The underclass of "scourers" also helps. The scourers swarm onto the streets when the gentry retreat into their mansions; they collect all the recycling, and they take any left over free items on the curbs and gather them to sell and make some money. The scourers are like a benevolent parasite, like cockroaches that scurry out to pick up the crumbs dropped from the cows fat on the riches of society.