I have been searching for a proper answer to this question, have a bit of background in front end development, but with new concepts like SaaS, PaaS, etc. want to get information from experts out there, that could help any newbie to understand what it's all about.

Say I am trying to develop eBay like an app that takes a product from a user and sells it back to the other user who needs it. 

Will my app need a backend server? If so why? I am already uploading my app to Google Play Store or Apple Store. 

How will the backend server like HEROKU or FIREBASE or AWS help my app? 

Can I implement two different services in single app, say for eg., firebase for backend database and HEROKU for payment processing? 

Thanks again for your time and information.