If you sell the timeshare at a gain (and I seriously doubt you will) then you will need to file a tax return.
If you sell the timeshare at a loss, then no need to file a return at all. Losses on the sale of personal property are not deductible.
Now, if after the sale you receive a 1099-S at tax time, then it doesn't matter if you sold at a gain or a loss. You would be required to file a tax return.
