Assuming you are not in Beijing right now to do face-to-face transactions, your best bet is to go to a LEGITIMATE ticket broker that has a track record of reliability, and order from them with delivery/pickup in Beijing. And pay with a credit card to get some protection against fraud and dissatisfaction. Note that even from brokers, some events are basically unavailable at almost any price...swimming being the most notorious and gymnastics, diving also very very difficult. Others, such as athletics, are being sold pretty much at face value, if the brokers have surplus tickets after they filled their preorders. Just try to avoid a broker that adds on a ridiculously high fulfillment charge or other bogus fee. And get an assurance that the tickets are actually on hand in their inventory, in Beijing. (I met with a couple of different broker reps from USA last week, and they have most of their tickets sitting here in Beijing already in their makeshift "war rooms.")