Firstly, how I can buy it?

You can access CB Auto Actions using a Professional or Developer subscription. If you need only CB Auto Actions you can buy it individually from our add-ons page below.

www.joomlapolis.com/addons

I am wondering if I can use the CB Auto Actions so as to retrieve data from a DB and then pass these data to specific K2 ITEM extra fields.

CB Auto Actions comes with a Query and Code action. So you can execute whatever SQL or PHP you want. In other words yes, but you'll have to code it. CB Auto Actions is also primarily designed to act on a trigger (e.g. after registration, profile edit, login, etc..), but it doesn't have to if you want it to be ran manually or from CRON.