I happen to have a Lefty neck for the banjo if you’re interested. - But it’s specific to a particular type of banjo.
For Sale: 1 gorgeous custom neck made for a 70’s model Gibson RB250. This was made for the 2 piece flange. This custom piece was ordered through a dealer at the time of purchase (I know this because the original receipt came with it). The owner played it a ton but took very good care of it. It will need new frets which should run a few hundred but will be very playable afterwards. Aside from the frets being worn, the neck shows no sign of damage.
The value is subjective but a healthy range is between $500-$700 as is. Of course, I’d take more. 
