What's the deal with Orochimaru and seals?
There was only a part of Orochimaru that was in Sasuke's body. I believe that the curse mark transfers a part of his chakra and thus a part of himself into the body of the target. 

It was the part of Orochimaru that was inside Sasuke that was sealed away by Itachi using the sealing powers of Sword of Totsuka. But a part of him still lived inside the others who carried the curse mark and his chakra was there with Kabuto, who absorbed Orochimaru after Sasuke killed his main body. 

Remember that Orochimaru was very weak when Sasuke revived him that he had to absorb his chakra from Kabuto to become somewhat normal again.



And it is not true that Orochimaru couldn't be sealed.The 3rd Hokage couldn't seal him just because he was too weak by then to pull Orochimaru with him. And it is not that Orochimaru can pop-out at will from anyone's body. Sasuke resurrected him. He didn't pop-out at will.