Hmm, never heard of that 'rule', i.e. guests having exclusive rights to sell their CDs. I guess there might be a few who would insist on that, but IME the support acts will flog their CDs as well as the main guests. I've never seen anyone complain about the arrangement. I would regard any attempt by the guest to restrict sales to their own CDs as protectionist and against the spirit of the folk movement (or something like that).