However, part of the answer depends on how you own it--tenants in common versus joint tenancy, for example. See, for instance, http://en.wikipedia.org/wiki/Concurrent_estate
If your portion is held as tenants in common, you can sell your portion to whomever you choose. It's a defined, divisble portion of the whole. That's not the case if it's joint tenancy.
So: See a lawyer.