I belong to a non-profit 501C3 organization that has been purchasing books for resale from a company for many years.

There are two problems with your inquiry. First, and most importantly, 501(c)(3) organizations do not have "owners." Second, if you "belong" to a 501(c)(3), it may be a membership organization that grants you certain rights as a member, or you may be misstating your relationship with the organization and may simply be a customer. In any event, in the absence of an agreement to the contrary (or a provision in the organization's organizing documents granting you certain rights as a member), a non-profit organization may generally refuse to sell to anyone.