Setting up payment and delivery is very simple and I've done it a bunch of times. I wouldn't underestimate the "average WP dev" either, they are getting a lot better than perhaps people on here may give them credit for. I can say the competition is certainly higher quality since when I started a few years ago but this has always been a side-business of mine alongside various startup jobs and other businesses.

The simple answer to why I use Codecanyon is access to a large, targeted customer base that is ready to spend money. That's not an easy thing to replicate. No matter the combination of PPC, content marketing, influencers and direct contact I try.