I know a lot of members (including me) have gotten great deals on craigslist. However, after my last experience, I won't ever buy an aquarium off of craigslist again (especially a large one).
I wanted to upgrade to a bigger tank, and because money was tight I though that craigslist was the way to go. Anyways, I ended up buying a 6 foot 118 gallon tank and stand. The guy filled it with water, we checked for leaks, and everything looked perfect. So I brought it home, stocked it with fish and was very happy for the first month. A month later, I came home to find about 10 gallons of water on my living room floor. The tank was leaking. I immediately had to pull all of my fish out, drain the tank, and somehow had to scrounge up the money to buy a brand new tank which was very expensive. After the whole experience was said and done, I had ended up spending more than I would have if I had just bought the new tank in the first place.
Something to consider: Old tanks have a greater chance of leaking than new tanks, especially if there is stress on the seams when you transport it. Large tanks also leak easier than small tanks because of the greater water pressure. If you buy a tank new, most tanks come with a guarantee, so as long as you use the tank properly and have it on an aquarium stand, if it leaks, they will replace it for you.
I don't mean to discourage you at all, but I just wanted to share my experience with you as a cautionary tail. Its possible to get great tanks on craigslist (I got my 80 gallon on there 2 years ago and its still holding up great), but there is no way of knowing for sure whether you will encounter problems.
Good luck with whatever you decide

New tanks are exciting for sure!