I got my first ever guardian bell today, thank you Juliana.
According to the folklore, The Guardian Bell is given to you by another person that cares for you. It is to be mounted low on the bike to ward of the evil biker gremlins that cause all sorts of issues on your bike and put you at risk on the road.
The Guardian Bell protects you while on the bike not the bike. And this protection is transferred bike to bike , thus if you sell your bike you should take it off and place it on the new bike.
Check out the link on different versions of the Legend of the Guardian Bell.