Speaking as someone who runs a nonprofit, make sure they want it first. I'm assuming that almost any organization in your area would simply be accepting such a unique in-kind gift to sell it or maybe auction it at an annual event. If they plan to sell it, they will have the same difficulties you did, only with nonprofit staff time tied up in it. If they can auction or silent auction it at an event, it may make more sense.

If I can ask, why have you had trouble selling?