The loss on this purchase is a short-term capital loss. You do not need to prove that it was for rental.
The maximum deduction for capital losses is equal to your capital gains plus $3,000. If you had no capital gains, then your loss is limited to $3,000 per year until you have depleted your capital loss carryover. You will need to keep track of this capital loss carryover, which TurboTax will do as long as you keep importing the previous year's return into your current year.
Make sure that you keep backup electronic copies of your TurboTax files to guard against loss in case of a computer crash. The only copies that TurboTax Windows automatically keeps are on your computer, not in the cloud.
This
article explains more about the tax treatment of capital gains and losses.
