Because they are divided among multiple individuals, timeshare owners rarely see an appreciation in property value. Therefore, timeshares are often thought to lose value over time. However, timeshare properties are not necessarily more prone to depreciation than other forms of property; but they are likely to cost much less on the resale market than they'd cost if purchased directly from the resort.
Because timeshare properties will not glean as much when sold back to the resort, owners are often encouraged to sell their timeshare resale. Timeshare resorts are often only willing to buy-back the property for a small fraction of your original purchase price. They'll then sell it to buyers at inflated rates based on the market value of the property as a whole. With resale, on the other hand, sellers may be able to glean more than the market value assessment would dictate, yet will still be able to sell their property to interested buyers for thousands less than they'd pay purchasing directly from the resort.