NB, use jquery !
var webform = $('#block-webform-client-block-18');
var unmarriedbutton = $('#unmarried');
var buyingblock =$('#block-block-10');
unmarriedbutton.click(function () {
buyingblock.fadeOut('slow', function() {
$(this).replaceWith( webform.removeClass('hiddenbox'));
});
});
Was too fast, i believe it's the way you select your object (getelementbyid) then you create a jquery object from it... -> use jquery API