What Vice-Captains can do:

It is important for both new and old captains a like to be aware that members you promote to vice-captain within your guilds have the exact same set of permissions that you do as a captain. This includes the ability to transfer ownership of your guild (they can even make themselves the new captain). For these reasons it is absolutely imperative that you know and trust the member you are granting the vice-captain position to. Setting a random stranger as your vice-captain may put your captain status and guild at risk.

As such, when selling your guild, you should know:

It's also important to know that should you decide to sell your GGN later on, you absolutely do not need to make your buyer a vice-captain before changing ownership to them. Doing this is a very good way to get your guild stolen from you before you get paid for it. The buyer ONLY needs to be a member of the guild they are buying to have ownership transferred to them.