If I relinquish the ownership of my timeshare, can I write it off as a loss? Do I write off just the purchase price or can I write of the interest and dues, too?

Unless you used the timeshare as a rental in addition to personal use, your loss would be considered a personal loss and would not be deductible.  The following provides additional information about timeshare property losses. https://ttlc.intuit.com/replies/5339387