Building a simple ios native app for buyers & sellers (marketplace)

It's very possible, but I wouldn't call it simple. A mock application may be simple to write up, but to have a functional marketplace that people would actually use is a whole other story. Mainly because of security concerns.

I totally understand. There is many services that provide security online. right?

There's a lot more to security than just having a "service" online (which I'm still not sure what you mean exactly), but allow me to expand. Anytime you will be dealing with user account, security and privacy is a big expectation. I've seen some small start-ups claim to be secure, but they were proven other wise. This goes from making sure that all of your databases are secure, to the encryption you will provide over the web when sending these requests back to your server. A service like this is most likely to have more "code" done on the server side than on the client side. This means that the server that will be hosting all of this content must correctly handle all of this information, while your mobile device really just allows input, and shows some output on the screen. Lastly, this isn't to scare you away, but just to give you a peak at what you should really look into when creating an application with any user data stored on a server. My best advice is to build it. Hit the speed bumps and road blocks, but ask questions and you will be able to get there. It is not easy, because if it was... everyone would be doing it!

What about using parse for backend service? I am driven enough to make it happen.

Ernest Grzybowski has written some great points. As for Parse, I highly recommend it. I will be releasing an Android Parse workshop here very soon, and the next iOS course will use Parse extensively. Check out the Parse Store app and tutorial on this Parse Tutorials page. They have sample code for an app that uses Parse and Stripe for an ecommerce app, which may be very helpful for you.

Wow, thank you Ben. Ernest brought up interesting points. He's great to have on this forum. He's been helping me since day one on here.