FORGET BUYING ANY SERIAL OR USED SYNO. NO NEED FOR THAT
you need to install an earlier version of the station, like 8.1, buy your licence and add it with internet turned off, it will give you an error but the licences will be added.
once you test licence is added, you can upgrade the station to the last version. once you do this, you won't be able to add anymore licences, fyi you can uninstall licences and add them again with different xpenology or start fresh.
another option is to install vmm package and install a virtual instance of latest dsm and latest surveillance station. in virtual dsm the unit is seen legit and you can activate any purchased licence.