From myext64.ini :
; Multisell filtering - if true, allows only multisells mentioned in last HTML sent to user
; in format action="bypass -h menu_select?ask=-303?reply=#" where # is multisell ID
; TO MAKE IT FULLY WORKING, YOU NEED SOME CHANGES IN HTML - OTHERWISE SOME MULTISELLS WON'T WORK!
; If you need to allow multisell that's not in format parsed by the filter, you can use this hack (example):
; <A action="bypass menu_select?ask=23000&reply=655" altaction="bypass -h menu_select?ask=-303&reply=655">Buy special products.</A><BR>
; If you need to allow multiple multisells, you can hack it this way:
; <A action="bypass menu_select?ask=23000&reply=655" alt1action="bypass -h menu_select?ask=-303&reply=655" alt2action="bypass -h menu_select?ask=-303&reply=655">Buy special products.</A><BR>
; IF YOU DON'T USE THIS OPTION, ANYONE CAN USE ANY MULTISELL!
; FOR EXAMPLE BLACKSMITH AND MERCHANT OF MAMMON CAN BE USED EVEN IF USER IS NOT REGISTERED IN SEVEN SIGNS!
MultisellFiltering = true
Blitzkrieg's post in Antharas Terror Skill was marked as the answer