Yeah, craigslist is VERY useful to the begining saltwaterist. you can buy live rock from 2-4 dollars a pound. very useful, considering the rock will have most likely come from a well established tank. If you dont want any coral AT ALL then you dont really have to worry about lighting yet. You have powerheads, for now.
If all you have is a tank, stand, and powerheads, there are definitely some things you are going to need. for starters
-HEATER: for this size tank, I would get a 500+ watt heater (get two if you can)
-SALT: some good ones are Instant Ocean, Reef Crystals Reef Salt, and Oceanic (although you are going to want to get more opinions on this one, im not a salt expert)
-REFRACTOMETER: an item used to measure the salt content of the water in your tank. you can also get an item called a hydrometer, but I would HIGHLY recomend the refractometer, so much more accurate and reliable.
-TEST KITS: kinda self explanatory. tests the aquarium water for different things, like nitrates, nitrites, Ph, ammonia, and such
-SUBSTRATE: Whatever is placed at the bottom of the tank, can be crushed rock, or sand. I would recommend sand, but it all just depends on what look you are going for.
-NET: For catching your fishies.
-BUCKET: needs WILL arise
-PROTIEN SKIMMER: I dont know if you need one quite yet, but you will need one at some point