You've gotta be really careful with those ads that seem like super bargains. I drove 30 miles to see a 100G tank and stand on sale for $80 and it was actually a 60 gallon. I also drove 20 miles to see what was supposed to be a 7 ft. long 210 G that was actually a 5 ft. 100 G tank. People overestimate the sizes of their tanks all the time on CraigsList, especially people who aren't serious fishkeepers.
Always ask them to measure the tank and get all the dimensions before thinking it's a great deal. You can't trust sellers on CL to give you an accurate volume size of their tank because many times, they're just guessing. It seems like this guy is just guessing the size of his tank too.