Question: I managed to find the shopkeeper once, shortly after arriving at Hengsha, however I'm no longer able to locate him. My inventory is full, and I'd like to sell off my extra items to clear up space. 

On your first visit to Hengsha, where is the shopkeeper located?

Answer: 
There are 4 Merchants in Hengsha (3 black, 1 white market).

* LIMB Clinic
* Lin Fu Ren - Hung Hua Hotel (ground floor)
* Pen Xin Hao - Alice Pod Gardens (southwestern storage closet)
	+ He later moves to the Youzhao district, in the south east (by the locker with the exploding barrels).
* Lu Pin Rong - Sewers beneath the Hengsha Port, accessible only from Harvester Hideout
There are three, I believe. One on the Ground Floor of the Alice Garden Pods (in one of the closets), one in the basement of the Huang She Hotel (The place with the "working girls").

The third isn't as easily directed to, but it's in the same district as the Harvester Stronghold, at the end of a dead-end street.

(Based on CyberSkull's answer above, the first and third would appear to be the same).