The last past years I have found some interesting good sellers on the net and eBay. Here you have some of the best G-Shock and Baby-G sellers I have met on eBay in no particular order:
MSGdistributors: Michael is a friendly guy from Brooklyn, NYC. He imports many Japanese G-Shock models that are rarely seen outside Japan.
bbtiesto: A seller from Hong Kong who a lot of rare older new and used G-Shock's in his shop and on auction.
Krazys: A great seller from Singapore. Sells all kinds of collectors stuff, but also newer models G-Shock's for good prices..
Ozziejapan: Elliot is an Australian G-Shock collector from Japan, "cleaning up" his collection. He is planning to go back to Australia, so he sells a part of his unique collection via eBay.
Watchesdeals: A new seller from Hong Kong. Is pretty new on eBay, but has nice models for a good price. Although they claim not to have many models on eBay yet, they will list more different and rare models in the future. Watches deals ships registrated with tracking number. I got my watches in approximately 5 working days form Hong Kong to the Netherlands, which is almost as fast as an European shipment.
Premierworld: A Hong Kong based seller, that sells a lot of watches, not only G-Shock. A lot of the auctions start at very low prices and his auctions seem to be very popular. I have bought three times from him, two G-Shock's and a Seiko Automatic.
Das-uhrenparadies: Katja Hildmann has always some G-Shock's and Baby-G's on auction. Sells within Europe.