You'd think that each team would have their own means of selling tickets, but they use Ticketmaster. There must be some benefit to it I'm not seeing.
Also, as previously mentioned. I believe it is Ticket Master and only Ticket Master that has an exclusive deal worked out with the Packers and Season Ticket holders. If you resell your tickets through Ticket Master, they will guarantee that the buyers, no matter what they do, can not get your season tickets revoked. Unlike what the Packers do if you or someone you directly give or sell your ticket to, gets booted for something, the Packers can revoke the season tickets.