[Editor's note: It turns out that I'm almost completely wrong about this -- check out the comments!]
Hitchhiking used to be commonplace. Now we think of it as dangerous, but studies have shown that one was no more likely to be a victim of a crime while hitchhiking than in any other situation. And it makes sense — when you do a favor for someone, are you usually looking for a way to exploit or harm them? Do you open the door for someone hoping they’ll let their guard down and you can pick their pocket? For at least a brief window in American history, that baseline expectation of human decency was enough to make hitchhiking an attractive option.
Nowadays, most of us wouldn’t imagine getting into a stranger’s car. The very fact that they were willing to pick us up would make their motive suspect — unless, of course, we know that their motive was to make money. Give us a smartphone app and a financial intermediary (as with Uber), and we’ll happily jump into some random person’s car.
Something similar is going on with Airbnb. Obviously their system allows people to make arrangements that would have been difficult if not impossible to make without an intermediary of some kind before the internet as well. Yet we have a kind of “natural experiment” insofar as Craigslist allows people to post and respond to ads for free and make further arrangements without anyone taking a cut. Lo and behold, it turns out that Craigslist seems “shady” and disreputable compared to Airbnb. (This result is of course overdetermined insofar as Craiglist’s total lack of filters has made it a hotbed for scammers and criminal activities — and yet.)
This is the strange thing about the “sharing economy.” In theory, we could always “share” things with each other at any time, but we generally don’t — unless some kind of combined financial/technological intermediary legitimates the transaction by giving us a fancy website and taking a cut (as the Craigslist example shows, the technological intermediary alone is insufficient). We don’t trust each other directly, don’t trust basic human decency. Instead, we trust money’s power to make people behave in the ways we expect. We only let down our guard when we know there’s a third party who’s skimming a little money off the top in exchange for making us feel like there’s someone to sue if things go wrong.