I wonder if the code below is what you want.
document.querySelector('#buy-now').click()
It seems some scripts have not been loaded on the page you mentioned at the time of DOMContentLoaded. Adding a delay solved the problem for me, that is,
// ==UserScript==
// @name script
// @namespace sc
// @include http://www.aliexpress.com/item/Free-shipping-100-Genuine-Original-P1000-Mobile-phone-data-cable-for-Samsung-P1000-P6800-P7500-USB/1477114758.html
// @version 1
// @description http://stackoverflow.com/questions/22633509/userscript-to-click-button/22684724
// @grant none
// ==/UserScript==
setTimeout(function() {
document.querySelector('#buy-now').click()
}, 1000)