RESTws POST List (text) field type Error
The JSON format for list (text) for checkbox type would looks like follows

{"title":"My Day","type":"page","field_days_closed_v":{"und":["sunday", "monday"]}}


I've tested this, and working fine.

Here field_days_closed_v is the machine name of the  List text field (checkbox type). similarly sunday, monday are the values of checkbox type so change them accordingly.

Hope this helps. :)