I am sorry for very late reply, couldn't do it earlier.
So I tried to solve the problem as you mentioned, except I thought it would be enough to create only one user - for ads posted by non logged-in users. It works - when a random guest posts an ad there's automatically a ghost author assigned to it. The showcases I made available only for logged in users so the problem disappeared.
As you remember: I applied views to the post templates: one which allows logged-in users to display their company's showcase post (if they have one) in the ad post and another which allows to display user's ads in the showcase post.
Now, when a non logged in user posts an ad, the view displays a showcase which does not belong to it. The author of the ad post is my ghost-author and the author of the showcase is some user who created it. How is t possible?
"Could you please share problem URL where you are listing your post same as author equal to logged in user" - I actually added a post author filter <<Select posts with the author the same as the page where this View is shown.>>
These are the views:
hidden link
hidden link
The above views are inserted in, adequately, ads template and showcase template:
hidden link
hidden link
What is important - the problem appears only when a non logged in user posts an ad, and I do not touch it. It disappears when I make changes to that post, e.g. when I change it to draft. When I publish it again, everything is fine. I keep making those changes because I don't want to keep it visible like this, but every time a new post is added, it happens.
So I created a test ad post for you so you can look at it untouched:
hidden link
hidden link
And as you will see the author of the ad is "autoghost". But if you look at the back-end of the showcase it displays at the bottom:
hidden link
hidden link
you'll see the author is different.
Whew, I hope I made it understandable...please help.
And yes, I use CRED.