Hello,

as you have the phone number of the new owner, it would be best to contact him and ask him to re-configure/uninstall Anti-Theft.

if you still remember your avast! PIN, you can also disable Anti-Theft remotely:

SET PROTECTION
Sample: 2222 SET PROTECTION 0

Will enable or disable Anti-Theft protection (0 = turn off, 1 = turn on)
Caution: By using this command you will
completely disable the avast! Anti-Theft system
thus rendering the Anti-Theft protection useless.

Be aware, that your SMS command will be visible if a) the user upgraded to android 4.4 meanwhile b) the avast! PIN is incorrect

Regards,
Werner