In Hong Kong Market Place, you can find almost all kinds of Chinese food. They have a large variety of vegetables, meat, seafood, snacks and health products that you may not find in normal grocery store. If you come from other Asian countries and you miss hometown food, this is the place you can get those ethnic food.
My suggested items are:
produce: different kinds of fresh mushrooms, Chinese eggplants
dry food: Nissan instant noodles, condensed sweeten milk (for making coffee), New Orleans coffee (you will find that much cheaper than normal groceries store)
seafood: fish paste, fresh blue crab
meat: short pork ribs
frozen: Hong Kong fish balls, octopus balls
Near some Chinese festivals like Mid-Autumn festivals and Chinese New Year, they also carry some festivals food.