<input type="checkbox" name="buy-item"  class="checkbox-buy" value="value 1" checked="true">
<input type="checkbox" name="buy-item"  class="checkbox-buy" value="value 2">
<input type="checkbox" name="buy-item"  class="checkbox-buy" value="value 3">

var dic = $(".checkbox-buy:checked").map(function(){
  return $(this).val();
}).toArray();

console.log(dic); // => ["value 1"]