@Kirstin21
The listing is on the official Airbnb website and is new. Therefore it has the promotion of 20% discount on it for first 3 bookings. Some dates are indeed unavailable, but it can be bookings or simply blocked dates by the owner. It does not apply a fee for additional guests (so for 1 or 10 guests booked it is the same price, which IMO is little odd regarding the low price level). But a bargain is a bargain ! It has the Flexible cancellation policy on it, so if you change your mind, you won't loose all the money.
Please read the houserules !
If you want advise form Airbnb:
Contact Airbnb - Community Guide [Updated]
best regards,
Emiel