Unless you know exactly where the clothes (or any other items) are being manufactured, in which case stuff can slip out to the neighborhood market, the chances are slim that genuine Western-branded merchandise will reach the gray channels. Personally, I only buy no-brand clothes in China, and only when I need them in a pinch. Some of that stuff can be great bargains and terrific quality (probably backdoor production as anacapamalibu mentioned); however, for me, it requires effort and not worth the time to seek them out.
Off topic, I don't expect the kind of bargains that I assume OP is referring to anywhere in China; however, in Bangkok's Chatuchak Market, I've found Ike Behar (one of my current favorite shirtmakers) dress shirts, still with Macy's sales tags on them, for ~ $6 to $10 each. But that stuff came out of bales that the merchant probably paid by the kilo without knowing what's inside or who Ike Behar is.