Like has been stated on this thread, just check the reviews etc. Oh, and I read recently that a common scam on AirBnB is when you contact the owner to book via the AirBnB site, but the contact you outside of the website and request deposits/payments outside of the AirBnB framework. Then when you show up at your accommodation, it either doesn't exist or there are people staying there who have NO idea who you are or about the AirBnB booking.
But, I see a number of people have used it successfully, so guess you just need to use common sense as with anything like this.
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