But doesn't that keep it jailbroken? I want to sell my iPhone on eBay unjailbroken but give the option for a potential buyer to jailbreak it by leaving it at firmware 6.1.2.
Apple isn't signing 6.1.2 anymore, unless 6.1.2 is the last version your device supports. I think, in order to unjailbreak, you'll have to upgrade to iOS 7.
I think the devteam are working on an iOS 7 jailbreak though.
But doesn't that keep it jailbroken? I want to sell my iPhone on eBay unjailbroken but give the option for a potential buyer to jailbreak it by leaving it at firmware 6.1.2.
Yes.
Leaves it jb still but completely wiped.
If someone wants it jb then it's perfect. If not all the new buyer has to do is plug it to iTunes and restore it to the latest iOS 7.