I am developing an app were people can post events and use a digital ticketing feature so users can purchase tickets to the event.
Can I use a marketplace like Braintree's or Stripe's or must I use Apple's IAP?
I've read the IAP guidelines but am still a bit unsure, I am more towards the side that I won't be needing to use IAP (too expensive anyways).
In this case you can't use Apple's IAP since the product is an event that is delivered outside the app. If they were purchasing access to an event stream that they viewed in the app, you would need to use IAP.
From the Review guidelines
3.1.5(a) Goods and Services Outside of the App: If your app enables people to purchase goods or services that will be consumed outside of the app, you must use purchase methods other than in-app purchase to collect those payments, such as Apple Pay or traditional credit card entry.
You can choose any other payment gateway you like, such as the ones you mentioned.
Note that Apple Pay is different to In-App purchase. Apple Pay requires that you contract directly with a payment processor that supports Apple Pay, such as Stripe or Braintree. Apple Pay simply makes it easier for users to use credit cards from their on-device Apple Wallet. With IAP, Apple handles the payment processing using the payment method from the App Store account.