I just sold 2 S3's with lifetime sub, so I know the drill.
The best and easiest way to do it is to phone TiVo, tell them you're selling a unit with lifetime sub and they'll give you a reference number which you can then give to the new owner once it's sold.
He/she phones TiVo and quotes that reference number which serves as your permission/confirmation and allows them to move the unit to the new owner's account.
The other way is to give the new owner your name, address, and phone number and they can quote that to TiVo, but my preference is the reference number.
Note that after the transfer you'll still see the unit listed under your account, it remains there as a static reference to the original owner, you can't modify it's name or preferences any longer (but the new owner can under his account.)