There are so many more subtleties to actually having a stranger come and stay in your house than there are to sending a stranger a book, guitar, etc via USPS.So, PG is a good salesman, and he hit the nail on the head with this comment:
Just wait till all the 10-room pensiones in Rome discover this site.
If you ever read a Rick Steves guide (which are not exactly the scariest guidebooks on the planet -- they're fairly comfortable middle-class NPR stuff) or tour around Europe in backpacker mode you will become familiar with the pension, aka the German zimmer or the English B&B. These are rooms. In pubs, or very tiny hotels, or people's homes. Which get rented out. They are very common, they have been a tradition forever, and they aren't very formal. Maybe you call ahead. Sometimes that doesn't even work. Sometimes you call ahead but then the landlord forgets your reservation. But ultimately you turn up, meet the landlord, get taken to the room, you look around for two minutes, you say "si" or "no" and fork over a night's rent, and there you go. It's not like buying a house or getting married or anything.
Yes, it's a bit more subtle to rent a room than to deal in used books. But it's not that subtle.