If your previously owned radio is still showing up in your SmartLink "Radio Chooser" Screen:
You probably left the FlexRadio registered to your SmartLink account when you sold it.
To resolve this situation, you can provide the new owner with these instructions to remove your SmartLink account association from the radio.
How to Completely Reset a FLEX-6000 – FlexRadio
Or ask the new owner to create his/her own SmartLink account and "Register" the radio to that new account.
ALTERNATELY: If you cannot contact the new owner, create a new SmartLink account and register your current radio(s) to the new account (and log out and log in to all of your devices with the new account). Then abandon the old account (and change the password on the account so that no one who had the old password can access your old radio).