the only time I've ever bought a tank on Craigslist, it turned out to be a terrarium for lizards and snakes. I filled it with water and it bowed out like a balloon. Not the seller's fault. Like most things, it was my own stupidity. Someone already said it, but it's a simple as knowing what to look for, like glass thickness, support, condition of seals and silicone, etc. I don't think you should stop looking or stop trying, but a lot of folks here can tell you what to look for. In my case, I bought a 40B Terrarium and a stand. It turns out that the stand is a decent aquarium stand and the tank wasn't suited to be used as an aquarium. I paid $50 for the tank and stand, turned around and sold the tank a week later for $55, so I made $5 on the deal and essentially got a free stand.
Good luck!
