The only problem as in finding what your calling true suppliers is that there really isnt any. Most designer clothes are purchased in heavy bulk direct from the clothing manufacturer.
And when you buy direct you have to sign an agreement with this company stating you will sell them at certain prices and not on ebay. Some of the biggest names force retailers to either sell them or destroy them to protect the value of thier brand.
When you buy these on ebay you are buying them from sellers that buy them in liquidations.