We are very clear with the issue, The "30" at the end of the url is the category id of the item that you had opened and clicked the "Publish new ad" link from.
If you disable the friendly url option from permalinks settings page, then you will see the different url with "catId=30" at the end of it,
which means category id is 30 of that item.
And this is default thing in osclass and won't give problem while posting new ad.
Hope you are clear now.