Checkbox enable readonly Input Not working

can anybody see why this is not working. it does not make the input field read only when the checkbox is not checked.

i have followed all the online advice so far and read much advice but even though i have followed what i could it does not work, however it throws no errors.

Would be great-full if anybody can see why, Thank you...

<div class='div1'>

<input type="checkbox" class='Xcheckbox' id="byname" name="searchforname" title='Search For All Listings With Sellers Name'  value="sellersname" onclick="chbxX(this); <?php $extrasearch==='sellersname'; ?>" <?php if ($extrasearch==='sellersname'):  ?> checked="checked" <?php endif; ?> ></input>

<label for="byname" class='nameheader' title='Search Date Range' ><b>Seller's Name</b></label>

<input type="text" id='sellersname' name="SellersName" placeholder="Sellers Name" class="sellersname" title='Type The Sellers Name' value="<?php echo $sellersname ?>"  ></input>

</div>


<script>
$(function(){

if($("#byname").prop('checked') == false){
    document.getElementById("sellersname").setAttribute("readonly", true);
  } else {
    document.getElementById("sellersname").removeAttribute("readonly");
  }


});
</script>