[Dilettante_Pro]

Может, так?

<div class="col-md-3 col-sm-6 col-xs-12">
   <select id="sdelka" class="sel" name="rent_buy">
      <option value="?">Sale/Rent?</option>
      <option value="0">Rent</option>
      <option value="1">Sale</option>
  </select>
</div>
<div class="col-md-3 col-sm-6 col-xs-12">
  <select disabled="disabled" id="pp" class="sel" name="type">
      <option value="">Property type?</option>
      <option class="hd1" value="1">Villa/Home</option>
      <option class="hd1" value="2">Condo/Apartments</option>
      <option class="hd1" value="3">Bungalo</option>
      <option class="hd2" value="4">Villa/Home</option>
      <option class="hd2" value="5">Condo/Apartments</option>
      <option class="hd2" value="6">Project</option>
      <option class="hd2" value="7">Commercial</option>
      <option class="hd2" value="8">Land</option>
   </select>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script>
$("#sdelka").change(function() {
        var v = +this.value;
        $( ".hd1").css('display', ['block', 'none'][v]);
        $( ".hd2").css('display', ['none', 'block'][v]);
       $('#pp').prop('disabled', isNaN(v));    
});
</script>