Remove owned modem from my internet account so I can sell the modem

Can customer service please contact me.  I've upgraded to use Xfinity's modem/wifi router and would like to be 100% certain that all references to the previously used Arris SB6190 modem is removed from my account so that I can sell or give it to someone.  I do not want the next person to have issues using it because the MAC address or serial is still somehow connected to my account even if it has not been used for over a year.

Xfinity internet hardware/support dept, can you please DM me so I can give you the modem information to be certain its removed?