Oh ya, I know Randy, the owner of Coral fish. You should also check out modern pet down in Waikiki. His prices are a bit higher but I know where he gets his fish from and they are nice stuff.
As far as the rock goes, yes you can sell river rock in Hawaii but there is really nothing on it. You can also play the grey area of picking up rock that has been washed up on the beach due to high surf. If you catch it right after the surf, you find lots of stuff on it still. This is known as coral rubble which you can collect up to a gallon bucket per day. That is the catch 22. You can only collect this for personal use and not to re sell.