After they'd rejected my first submission, saying that my documentation was a little lacking, I added a Quick Start Guide along with some screenshots. Apparently, it was enough to get through the second round of reviews. You can read my thread on the Envato forum to get more helpful tips from other people who got their submissions rejected:
So, what does my app do? It lets you transform Facebook albums into a beautiful photo gallery. Just pick a Facebook page (it can be your own profile) and the albums you want to show in the app (you can filter them by name, id or date). Photos will get synchronized with the app and they will continue to stay up to date, displaying a notification whenever new ones appear.
As to the technical details, the app uses some pretty slick stuff like SyncAdapters, Material Design, Immersive Mode, Facebook Audience Network ads, Facebook Graph API, RxJava 2, Moshi, Glide, JUnit 4, Mockito. It might be worth buying just for the sake of learning how to use some of those libraries.
Here are some promotional images, if you're lazy enough not to go to CodeCanyon to actually see my app there: