Possession and transfer of certain specialized firearms are strictly regulated under the NFA. An FFL holder must have a valid SOT to deal in these items.
Your SOT document must be added to your Master FFL Profile, along with your FFL.
Follow this link to Claim your profile with Master FFL and to add your FFL and SOT.
If you have questions about Master FFL, please see GunBroker.com Partners with Master FFL and Common Master FFL Dealer Profile Questions