With jQuery:
// on page load
$( "#buyerForm" ).show();
$( "#developperForm" ).hide();
// radio button actions
$( "#buyerButton" ).click(function() {
$( "#buyerForm" ).toggle();
$( "#developperForm" ).toggle();
});
$( "#developperButton" ).click(function() {
$( "#buyerForm" ).toggle();
$( "#developperForm" ).toggle();
});