Go to the first post in your thread
Click Edit at the bottom right corner of the post
Click Go Advanced
Click at the beginning of the title and simply type *SOLD*
Click Save Changes
Then just post one more time in the thread stating that it is SOLD and you are done.
That's it. Don't delete the body of the thread. Don't delete the title. Don't try to get creative by typing Gone, Delete, Solded or anything else. Simply add *SOLD*. There is no need for you to report the post if you do it this way. Thanks