Patience is the key on Craigslist. Last month I picked up a 55 with HOB, heater, hood and lights for $25. Try the keyword search using fish, aquarium or gallon(one at a time) in furniture, general and household. My 25 dollar deal was found under household, not pets.
If it says best offer, lowball. Worst they can do is say no, they may even say no at first, but when the offer above you falls through, they may re-contact. I've had that happen. If you aren't too eager you can get amazing deals sometimes. Leaves you more money to stock it.