When creating a new field for the profile, you'll use the field's name: "purchase_details" (the label can be whatever you want) each user can edit this field when editing their profile and fill there the contact information that can be used for the other party to contact him (those can be bank transfer details or any other information for the seller/buyer).
This field will be visible only for the administrator or the author of the advert. So it's important to set the field to be visible like this.
Here's the screenshot of how the field should be set up:
Now, when a user edits the profile - the new field is visible and ready to be filled (on the screenshot you'll see exemplary filling):
When filled, the author or administrator will see a button to trigger the popup with that info in their History of sales and orders:
If the user doesn't fill the field, it'll display only username + email.
