From my experiences, most of craigslist (personals, employment, for sale, housing, etc.) is a complete crapshot. Some really straight-foward honest people, some scammers, some deluded folk, some difficult people, some staging muggings, etc.
As to aquarium-specific, it seems to be all over the map. A lot of overpriced tanks, but occasional deals as well. A few months ago I ran across an add where someone was selling a tank with 20H dimensions, but claiming it held "at least 50-60 gallons", and wanted well over $100 for it.
Reading the 'best of' can be pretty amusing, and give some insight on the types of people that frequent craigslist.