Detect when a specific <option> is selected with jQuery

$("option#trade_buy_max").change(function () {
    opt = $(this).children("option:selected").attr('id');
    if(opt == '#trade_sell_max'){
        // do stuff
    } 
});

Untested, but that should work.

you can bind change event on its select instead, then check if option selected

$("select#type").change(function () {
   if( $("option#trade_buy_max:selected").length )
   {
     // do something here
   }
});

This works... Listen for the change event on the select box to fire and once it does then just pull the id attribute of the selected option.

$("#type").change(function(){
  var id = $(this).find("option:selected").attr("id");

  switch (id){
    case "trade_buy_max":
      // do something here
      break;
  }
});

What you need to do is add an onchange handler to the select:

$('#type').change(function(){ 
  if($(this).val() == 2){
     /* Do Something */
  }
});