How to create a view block only visible to the Referenced Content Type's Author?
Easiest way to do this:


Add a content relationship for the node reference field (i.e. Content: field_item_for_sale_ref)
Add another relationship for the Content Author and select the node reference field relationship from the dropdown list at the top of the window.
Filter the view for the current logged in user by selecting the author relationship that was just created in step 2.


OR


Add a content relationship for the node reference field (i.e. Content: field_item_for_sale_ref)
Add a contextual filter for the Content: Author uid, select the node reference field relationship from the dropdown list (top of window), select: provide default argument of type: user id from logged in user


Either way the result is the same and it's what you need.