A snippet (using Prototype.js as available on the page) to instantly buy the top item in a Steam market listing. Can be used as a bookmarket.

SteamMarketBuyNow.js

$$(".item_market_action_button.item_market_action_button_green")[0].click();
$$("#market_buynow_dialog_accept_ssa")[0].checked = true;
$$("#market_buynow_dialog_purchase")[0].click();