How do I get an iPad ready for sale

I have an iPad Pro 128 GB Wi-Fi + LTE (one of two), running iOS 9.2.1. I bought two, one to use, and one on which to test an app I wrote. Now, I can see having the second one is superfluous. With the new Xcode, and the ability to prototype in browsers — I do not need to have so many actual devices, I can virtualize this. So, I am attempting to release bench test devices into the wild.

I have unsuccessfully tried to sell iPads, iPhones, and other Apple devices on eBay. Inevitably, instead of just putting it on there, selling it, and being done with it; I relentlessly have to follow up... and I am not great with people 😐

If anyone would be so kind, as to give me a contemporary (2015/2016) set of instructions — I need to know the precise steps to take, to completely set an iPad with Wi-Fi + LTE for the next person who will be purchasing it. I would like to know how to set-it-and-forget-it. It's somewhat maddening having a collection of unused electronic devices that are contemporary, powerful, and useful — just not to me, and so much of a pain to sell, that it had his remained not worth it to me to sell them.

I realize this is not a deep technical question — but finding any current, reliable information on Google is almost as laughable as the fact that Apple does not have an app, tied to your Apple ID (including touch ID), that would allow you to completely "reset for sale" an iOS device in one easy step.

Thank you in advance for your time, and talent!