Looks like a standard 20 long to me. Why would it not be designed to hold water?
You could get a new 20 gallon for that price. He's asking a reasonable price for what it is. It doesn't come with a filter or full light strip or anything else you'd see normally included if you were buying a used aquarium. I'd keep looking I'm sure you can find a similar deal on a tank someone used for fish that would have the equipment you need with it.
Malaysian Trumpet Snails: JUST SAY NO!