Learning to 'backdoor' e-commerce sites

Hi,

I'm a sneaker buyer/seller and I've been reading lots about checking out of sites early or having early links to sneakers that are not even live pages yet. My question is how does one start with finding things such as this and what sort of tools are required. How do you go about finding a link that is not even live on a website yet for a product set to release in the future and then checkout with it? This tends to happen on all sites (minus Nike) and is very prevalent on small boutique shops . I've tried some very basic data-mining but typically come up with garbage as I'm only knowledgeable enough to crawl live pages and links, what is a good resource to learn this type of thing?

My experience is somewhat limited in this realm but I have some very basic coding experience.

I highly doubt someone is going to share this on public forum. And besides that, what this have to do with blackhat seo?

Look at robots.txt, search the site with Google Advanced Operators like site:website.com, Google Dorks, URL fuzzing, directory browsing, directory fuzzing.
So many ways of doing it.
Read up on web application security testing.
https://www.owasp.org/index.php/Main_Page
http://www.googleguide.com/advanced_operators_reference.html
https://www.exploit-db.com/google-hacking-database/
https://en.wikipedia.org/wiki/Google_hacking