How do Chinese eBay sellers ship so cheaply across the world?

YES, there is a "trilateral agreement" between the USPS, eBay and the China Post, whereby small items can be shipped more quickly and cheaply, along with a couple of other pro-Chinese mailing methods. [ref: http://www.ecommercebytes.com/ca... ] China Ebay

The other answers are also affective in the broader sense: Chinese monetary policy does keep the Yuan artificially cheap, with attendant benefits like comparatively cheaper postage;  the "slow boat (from) China" transport shipping is shipping with enormous economies of scale, making long haul slow shipping less expensive (and all the ships returning do not return stuffed to the gills as they arrived - monetary policy is not solely responsible for trade imbalances, as was suggested); and China Post is subsidized by the government, of massive scale, and protected from competition.Taobao agent