Sand sifting starfish eat the worms, pods, living organisms in the sand for food. That is all they do all day long, is search for food.
I agree with and without the assumption not to own one.
Thus being I have had one for over 2 years in a sand bed of 3 inches deep and it has more than doubled in size, and I stil have plenty of life to go around in the sand.
But you have to have a stabile and mature tank since they are very sensitive to water quality etc. Plus the tank they would be going in has to have plenty of liverock.
The sand will become live with all the worms etc after a couple of months since the liverock will seed it and help it gather the critters.
Buying sand with the critters in it wont happen unless you buy sand form another reefer or your LFS that has had the sand already established in their tank for sometime.
The Live sand that you can purchase only contains bacteria and is a waste of money, you wil get the same affect from the live sand for sale, as you would with liverock in the tank after a few months with base or dead sand. The liverock is what actually holds all the bacteria, and from that you will start to aquire the living organisms in the sand.
I would wait atleast 6 months before adding a starfish or anemone. Thats is of course if the lighting you have is strong enough to keep the Anemone alive.