How to get iPhone 5s as upgrade then sell it and keep current phone?

I have a dumbphone right now but I only pay like $20 for unlimited talk + text. So i'm due for an upgrade and want to get the 5s but not activate it and sell it then continue using my current phone. I heard that you have to add a data plan but that after it's added to your plan you can just cancel the data? How do I handle the activating sim situation. Will it disable my current sim and if it does how do I reactivate it? If I say keep it and use it like an ipod touch can I keep the sim in it and turn off cellular or will I have to take the sim out?

Added (1). But what if I activate it to sell it how do I do that?

You can't activate the iPhone unless you are the person who purchased it.