Really?
Whenever I've done the comparison shopinhk has generally, on average been slightly more expensive than Amazon or Bookdepository, but to get "a couple of hundred dollars" on "a couple" of books implies about $100 per book, which would be about 50% of the price.
The first ones I checked (Lonely Planet Japan & Croatia) were HK$36 and HK$12 cheaper on the Book Depository than on ShopInHK at interbank+3% rates. You'd need to buy more than "a couple" to save "hundreds of dollars".