Copying up a comment I just posted following up on someone's review thread, as I think it's a good general thing to consider up front. People who haven't been deeply involved in the Apple ecosystem often don't understand how strict the App Store rules can be.
In their case, my concern was triggered by this headline on their landing page
tournament and trade platform that offers ... and cash prizes
My comment in the thread:
You have to be very careful when cash is involved.
The area I'm pretty sure covers you is:
5.3 Gaming, Gambling, and Lotteries
Gaming, gambling, and lotteries can be tricky to manage and tend to be one of the most regulated offerings on the App Store. Only include this functionality if you’ve fully vetted your legal obligations everywhere you make your app available and are prepared for extra time during the review process...
Apps that offer real money gaming (e.g. sports betting, poker, casino games, horse racing) or lotteries must have necessary licensing and permissions in the locations where the app is used, must be geo-restricted to those locations, and must be free on the App Store.
Every developer and product manager with significant business plans for iOS apps should be checking them against https://developer.apple.com/app-store/review/guidelines/ early in the process - you don't want to have to fundamentally change your entire business plan! As general advice, at WWDC time (just passed in June), Apple have labs where you can ask business model and marketing as well as tech questions. I am certain they also have ways to ask these questions at other times but haven't had to do so and so don't know the avenue.
https://www.appstorereviewguidelineshistory.com/ is the best site to clearly see how the guidelines have changed over the years, which lets you see trends and reversals in Apple's attitudes.