Users do not have the power to delete an entire thread, but you CAN edit the original post. Just click the "edit/delete" button near the bottom of the post box, and when the edit window opens up, erase everything in your post and replace it with "SOLD" or "No longer for sale" or whatever. Then click the "Save Changes" button, and that should take care of the problem.
“It is foolish and wrong to mourn the men who died. Rather, we should thank God that such men lived.” -- George S. Patton