The following returns selected items correctly from standard list and document library types.
var context = SP.ClientContext.get_current();
var items = SP.ListOperation.Selection.getSelectedItems(context);
on the external list it seems to only return the list object itself as the ID returned is that of the external list "__bg01003300730033001300" instead of the list item ID"s.
Is there an alternate method when dealing with External lists? I would not think so since we are still dealing with listview (and I have not found such a reference).
I have hit a road block in terms of other items to verify, I tried from both Chrome and IE with same behavior and on two different farms.