The problem may be that the craigslist seller purchased the tank used and was just telling you the size they were told at the time they purchased it. It's unfortunate when people sell things they know nothing about, but it's just part of life. The best thing you can do is to protect yourself in future purchases by knowing where to quickly find answers to verify what you're purchasing. Figuring a rectangular tank size is simple, hexagonal...not so much. For a rectangular tank multiply length x width x height, then divide that total by 231.