My understanding was that a lot of the produce sold in Chinatown markets was near expiration dates and often had already been rotated off of the shelves of normal supermarkets, often for cosmetic reasons or as a way of segmenting the market.

(basically, supermakets sell produce to Chinatown vendors for a lower price, who then move it to a segment of the market which is willing to buy non-aesthetic food which needs to be eaten in a day or two)