Apple Unlocking iPhone 4s on Sprint?

[Tom Bombadildo]

So my family is upgrading their phones, and I'll be getting a couple iPhone's they want me to sell online. While I could just leave them locked to Sprint, I'd like to unlock them both (as I'm selling them online). I was wondering how (if you can) you unlock an iPhone 4s on Sprint. I've tried a bit of googling but didn't seem to find any good answers, so I figured I'd check here as a bit of a last resort before I just sell them locked. I don't have either phone right now, so I can't give any version info or anything.

Thanks.