Besides what’s been already said:
You make more connections as time goes on, and you also learn how to detect deals better. For years I’ve been buying inventory form old school sellers who have websites that are impossible to find through normals search, due to their web 2.0 designs, and I’ve closed at least five of them down because they undersold product to me/I’ve cleared their inventory and I know it was me because I made the purchase and the next day they’re offline. Basically, once a year I go onto their sites and since they haven’t updated any of their prices, cards that were market price last year are now underpriced, recently I’ve bought a base box for 4k and a 1st ed fossil box for 3k. Some of the site owners I still have contact information on and can email them for some cards. Very few of these sites are still active, and most of these sites are pretty much sold out now. No, I will not share any of them or my search methods for finding them, my list of these sites is very short now.
The more practical way is to find someone selling cards on eBay and messaging them asking if they have other cards you look for. The majority of people only list the higher ticket items because they don’t have the time to go through their entire collections and sell off every card, but will happily sell what you’re looking for if they have it. This is way more effective than trying to search for the needle in the haystack old school pokemon site and is honestly my go-to method.
The final strategy is just to be first to eBay. I assume this person was told they undersold their strong 9s-Gem Mint 10 cards to me because they told me after they shipped that they wanted the cards back. Of course, I didn’t send them back mainly because of their messages to me. www.ebay.com/sch/enol_5/m.html?_nkw&_armrs=1&_ipg&_from&LH_Complete=1&rt=nc&_trksid=p2046732.m1684