I don't know if this is an issue related to the Osclass or to the Delta theme
If i want to delete all the listings the last listing in the item list still remains.
It is still available on on search.
If i try to delete it the message appears : The listing you are trying to delete cannot be deleted , then it redirects me to the first page.  ( or something similar since iv translated the theme )
On the front page there is no ad, only on search.
If i try to edit i get the message : We are sorry, there is no listing with this ID ( or something similar since iv translated the theme ).

@StefanGr
This is usually caused by foreign key block when removing item.
Try to go to database and remove entry from t_item table, you will get error with details what table/key is responsible for that.