Are you selling the disks with the system?
If so, see the article on System partition, volume and file system information for the SS4000-E. There you can see that the operating system is resident in flash memory on the SS4000, the flash operating system is copied to the /dev/md0 partition on the storage system array and runs from there.
From that we know that the writeable data is on the disks. If you want to "sanitize" the system for a new user, I'd suggest deleting all the existing data and selling a new clean system. This will include data, user accounts, shared folder configuration and system logs.
To do this you can pull each individual disk out of the SS4000 and install in a PC and format the disk. Do this to all the disks and then reinstall into the SS4000 and reinitialize as you did when you first set it up.