Dear Adriano
So, I am still not understanding exactly what you mean.
I am sure that the objective of the function is to get the parent post status. If the parent post status is 'private' then this indicates that someone else has already purchased the item and so the action of pressing the 'Place Order' button (the CRED form 'Place Order' must be halted and an error message displayed.
The CRED form used to create the 'Place order' function is used in two situations;
1. In a wpv view which displays the current cart contents. The url for this view is http://audio-gear.org/cart-listing/
2. In a single post view template which displays any one item in the cart. This has the url of the form http://audio-gear.org/sell-list/b772757/?view-template=Single%20Item where 'b772757' is the id of a specific item within the cart.
Let me know if this is enough information? If not then I can send you an admin log in for my site.
Kind regards
Robert