eHobbyAsia seems to be the largest Hong Kong based airsoft retailer on Ebay (listing over 4300 items in their store with over 28000 feedbacks). I've only placed a few orders for some knock off tactical gear (Helmets, Helmet Covers, etc).
They carry all the major Japanese brands of airsoft weapons and accessories. Most of the stuff you'd see at palces like WGC or Dentrinity.
The ordering was eBay easy, tracking number was sent a few days after paypal and items arrived when promised.