i remember a lady on creigslist one time did the same thing but worse. her exact words were
"this tank is HUGE and i need it gone!! only $2,000 for the whole set up!! it is about 6 feet long, almost 2 feet tall, and over a foot deep!! i figure it must be AT LEAST 750 gallons of water!!"
im still laughing today lol
but yeah just ask for the dimentions and convert it to gallons. most people selling tanks like that either lie hoping for a sucker, or just are flat out idiots.
Getting dimensions doesn't always work. I found a 5' 120 on Craigslist once for $100. I asked dimensions and they provided the correct ones. I went to check the condition and it was a 75. I asked about the dimensions. She had just looked it up online since her tape measure was missing. Luckily it was in town and only 15 minutes away.