Taobao uses a Instant Messaging system for buyers to contact the seller. Make sure you are logging in Taobao with Internet Explorer because that is the easiest way to install their Addon.
Here is what you will see in a search result page:

Here is what you will see in a individual detail page:

Click on the red circled orange button to continue. If the seller is offline, click the blue circled grey button to leave him/her a message.
Here is what you will see after clicking the button:

Chat with him/her the way you are using Windows Live Massager. Most sellers, I guess, do not understand English.
Please note in this way, taobao won't record your messages with the seller, and there are hardly any buyer protection if there are disputes. You can download their clients to better protect yourself (Buyers ediction:
http://www.taobao.com/wangwang/2010_fp/buyer.php?tracelogww2010=buyerclick), but I have no experiences with that. One thing to remember, do not open any document the seller sends you through the IM system on any circumstances. 90% it is a virus.
Good Luck with your purchase!
Note: Members in Mainland China may not view the images properly because imageshack is blocked in China. Use a proxy.