The way I've always done it (over 30 years, 3 co-owned airplanes, and many
different partners) is as follows: Each co-owner is free to sell his/her
"share" for whatever amount the market will bear. However (and this is a
HUGE "however") the remaining co-owner(s) each have a veto right on the
prospective new partner. Approval of a new partner is not to be
unreasonably withheld -- in other words, any veto has to come with a
reasonable justification. Also, if the new partner causes any increase in
insurance premiums then the difference is paid by the new guy.

HTH


-Elliott Drucker