You might want to read the T&C of the contracts you signed when you purchased your Westin, Sheraton or MVC timeshares. I simply pulled this wording from a Westin contract:
"8.1 Personal Use; Commercial Purposes. Use of the Units and facilities associated with the Network is limited solely to the personal use of Network Members, their guests, invitees, exchangers, and lessees of Home Resort Vacation Periods, and for recreational use by corporations or other similar business entities owning VOIs. Purchase of a VOI or use of Units and facilities associated with the Network for commercial purposes including without limitation rental purposes, for contribution to or use in a different vacation ownership plan or vacation club (except as expressly permitted in the Network Documents), or for any purpose other than the personal use described above is prohibited. Violations of this Section may result in the suspension of a Network Member’s reservation, cancellation of reservations for third parties made by Network Member, and/or suspension of a Network Member’s ability to make future reservations except for reservations of Member’s Home Resort Vacation Periods."