page purge

Hi,
I hope you are doing well.

I’m not a developer so before installing the Nginx cache purge module by Frickle (and do things that I’m not so comfortable with), I’d like to ask if the auto-purge is deleting the entire cache or just the edited, changed, deleted page’s cache only.

What I’d like to achieve is:

Whenever a customer buys a product, the stock number of that product decreases but because that product page is cached, it never refreshes and shows the actual stock number until I purge the entire cache.
I’d like to keep product pages cached because the speed difference between the cached and un-cached loading times is huge (25X to 50X)!

Thank you for your help.