The big secret is taobao! And to see if you can trust the seller, click your way in to the product page and look for the diamonds (). These diamonds and the % below them are different credits, the bottom % i think is how happy the buyer are with the seller.
I'll use the example of some of the most popular (and best, they say) supplements for building musicle:
BSN is behind no-xplode and a bunch other popular products.
List BSN products
The #1 protein due to popularity.
List Optimum Protein
A taobao translation plugin for Firefox would be awesome, but for now you can use http://babelfish.yahoo.com and just enter some page's url and select Chinese Simplified - English. A bit chingrish, but you'll understand most of it :)