Maintain selection coordinates using a closure. This example using jQuery.

Box Coordinates

var boxCoord = function(endCoord) {
  var coord = endCoord;
  return function(anotherCoord) {
    return [anotherCoord, coord];
  }
}

$('body').mousedown(function(e){
  var thate = e;
  $(this).mouseup(function(e){
    var box = boxCoord([e.pageX,e.pageY])([thate.pageX,thate.pageY]);
    console.log(box);
  });
});

mouseup.box is namespacing. That way when you unbind it, it only removes the handlers specifically registered with that namespace, mouseup.box.