How to use Drupal Commerce to extract all the people who purchased a specific product? I want to create a report (a list) of all the people who purchased a specific product.

I want to use this list for a report to verify purchase when people pickup registration packs etc., so that I can confirm those people when they arrive.

How do I get this in a report format that I can extract from Drupal Commerce?
Not 100% sure what you mean with "reports on line items" or "a line item report" (consider refining what actually this means). But since the question is tagged with the reporting tag, maybe you may want to consider the Forena module. 

It would probably take a bit of time to achieve perfection, but you'd have all of Forena's xHTML-power available to finetune the ultimate result (report), for things such as:


look-and-feel (styling/theming).
maybe add some charts to it also (using selected data about commerce).
saving the ultimate output (report) in formats such as PDF, DOC, etc.
creating eMail campaigns about selected records.
pulling in information stored in other databases related to this report.


For more details about Forena, 2 types of documentation are available:


Community documentation.
Documentation that comes with Forena, which you can access right after install and enable of the module. Checkout the demo site for an online example of the current:


Forena documentation - use the link 'Reporting documentation' or visit relative link /reports/help.
Forena samples - use the link 'Reporting samples' or visit relative link /reports/samples (these samples are fully functional, so make sure to experiment a bit with it, such as the drill downs available on the SVG Graph sample).



The newest 7.x-4.x version also includes an amazing (I think) UI for either creating your reports (the WYSIWYG report editor) and/or for creating your SQL queries (the Query Builder).

Here is some more info about the other databases in mentioned above:


Using a SQLite instead of MySQL database: the Forena samples actually get shipped including a (Tiny) SQLite database. Go check it out in the demo site: the data shown there are data contained in the sampledb, which is in SQLite format.
Forena comes with a full suite of Supported database connections ... such as MySQL, MS SQL, Oracle, Postgress or any PDO compliant variation.


Enough reasons for considering giving Forena a try? While doing so, use it's issue queue for any type of support/docu requests you may have.

Be aware: I'm a co-maintainer of Forena.