Here's a tip that I hope those who live near me don't see. I advertise in the local classifieds around the holidays and tax time; "Aquariums wanted, 100 gallons or larger. No breaks, some cracks considered. Paying $1/gal."
More than 1/2 the tanks I got were full setups for a buck a gallon. I check the dimensions before paying and argue the size if the tank is smaller. A few times, I got a larger tank for the cost of a smaller tank because the seller didn't know his 180 was actually a 240 or a 250 was actually a 300. And, frequently, sellers would agree to sell me a tank provided I also take every other smaller tank for free. I donate the small tanks to the local elementary and middle schools.