You should always do a mock return so as not to confuse anything on your real return. You can do it in the online version and then abandon it. The sale would be on Form 4797. You can delete it. You may just want to delete everything to do with Schedule E and redo it from scratch.
Here's the general procedure for viewing the forms list and deleting unwanted forms, schedules, and worksheets in TurboTax Online:
Open or continue your return in TurboTax.
In the left menu, select Tax Tools and then Tools.
In the pop-up window Tool Center, choose Delete a form.
Select Delete next to the form/schedule/worksheet in the list and follow the instructions.
