Have the listings you are looking at, had reviews from previous guests and are they positive?
How long have they been hosting?
Do the owners of the listing have a completed profile - ie full face photo, passport or driving licence and other ID?
Some areas to look at to help you make a decision.
Whereas long term lets are more unusual particularly for a year or more, they do happen.
If in doubt, get in touch with BnB directly about a listing to verify all is okay before you commit to booking.
For such a long term listing you might feel safer renting through an established lettings agency.