Check out Free cycle too!! Aquariums are listed there for free as well.
I live in smallish town that isn't listed on Craig's list, but freecycle is.
I check every day for deals on aquariums in nearby towns.
I was lucky to score my 125 gallon setup on EBAY.
I was always looking at tanks on ebay, but all were "pick up only" and thousands of miles away.
So one day I'm looking and an auction was ending in 36 minutes for a 125 gallon tank.
Low and behold......it was only 10 miles away, out in the middle of the country.
I couldn't believe it. 200 bucks for tank, stand, filter, sump, overflow, lights and refugium and then he gave me a complete 30 gallon set up too!
Used tanks are the way to go if you don't mind putting adding some elbow grease to save some cash.