Can I buy a used iPhone 5 from eBay and use it as an iPod Touch? I'm planning on buying a second-hand iPhone 5 from eBay and then using it as an iPod Touch (when the time comes, I'd like to activate it and use it as an iPhone). I'm not sure how activation works, however, and a few hours of internet searching have yielded several different answers. So far, I've heard that I'd need to get a SIM card matching the carrier of the iPhone, but I've heard some people say that it has to have been used with an iPhone before that?

Sorry for the possible triviality; I'm not too well-versed in Apple products and things. Thanks a ton in advance :)
The phone will need to be activated using a valid SIM card. If the device is carrier-locked it will need to be a SIM of that carrier. However you can just take out the SIM after activation and the phone will work fine until you reset it (then it would need to be activated again).