Hi, I am both a college distance runner (at a Nike school, no less) and a casual sneaker/streetwear reseller. I therefore feel I am qualified to speak about this topic.
When you are able to resell shoes at a higher price than retail, you are getting that premium for one of a few reasons:
1) You have a pair from the latest limited release that people who were unable to get a pair from Nike still want.
2) You have access to a pair prior to a shoes drop date.
3) You are able and willing to secure a pair on drop day for someone else, thus eliminating their need to stress about whether or not they'll be able to get a pair. Many such proxies use online bot services to check out faster than any human ever could.
4) Long after a shoes release date, you have access to a colorway that hasn't been re-released. Case in point would be the Off-white Nike Zoom VaporFly and VaporFly 4% Flyknit sneakers. Designer Virgil Abloh has stated that while he will continue to work with Nike, these shoes won't ever be re-released. A deadstock pair therefore goes up in value by the day.
So if Nike begins to saturate the market with releases large enough such that everyone who wants a pair can just go to Nike and pick out whatever color in whatever size at any time, the resale market will dry up for the 4%. Otherwise, it will still be there.