Since the Chinese merchants do not have e-commerce sites the ordering is done either via proxy such as eBay or directly by using a combination of email and chat. As it stands it seems to be best to steer away from dealing through other sites as AliBaba Direct due to a history of elaborate scams conducted there. The merchants do offer cheaper prices for group buys where customers (eg. from cycling forum) join in to committing to buy a minimum of 15 frames. While it has its risks and downsides the most common and the preferred method is still to communicate directly via email.
Email discussions can drag on for several weeks so use of skype or messenger is a good way to keep the ball rolling. You get immediate feedback to all your silly questions and then just do the official orders via email to create a paper trail. Basically once you know exactly what you need they will quote you a price and give paypal account details to pay to. Once you have paid it is probably a good idea to ask them if the transaction has come through just to be on the safe side. If all goes well in few days you will receive your shipping tracking id via email sent through paypal.
So in theory the ordering is really that simple. In practice there is the danger of scammers hijacking the email accounts or websites of the vendors. You may also end up haggling about the price for quite a while. Due to cultural reasons and due to the fact that the merchants themselves are not manufacturers it can sometimes be difficult to get them to say no. They will just repeat their previous offer after talking with a superior or the factory owner. Anyways it really pays off to have a proper list of all the items you need and price for each item. It is easier to compare prices for common parts and make sure you have everything.
- derailleur hangers
It is a well known fact that there are some shady practices in the international business. Merchants willingly offer low value shipping invoices to lower the income tax or other customs duties. It has also been implied on discussion boards that people have also actually marked the goods as something else or claimed the source country as Taiwan instead of Peoples Republic of China. Obviously such practices are subject to penalties and the risk of losing the frame is hardly worth the possible gains.