The best places for seafood buffet IMO are all in Macao, which is an hour away by ferry. Just visit any of the major casinos there and you will find a decent buffet restaurant. Prices range from US$20+ to 40+ per person. One suggestion is Sands Casino, which is located right next to the ferry terminal. Venetian is also pretty good but it's farther away.
If you are looking for other types of restaurants in Hong Kong, try the website www.openrice.com
That's what many locals use, but unfortunately it's available only in Chinese. Hope this helps!