I dunno if anyone here remembers but almost a year ago, someone was selling a 155 bowfront tank and stand in oak trim (I think) for $200. Absolutely NO takers for about 8 months. I think the guy had it in his garage and lots of people were fully committed to buying it so long as he would water test it in his garage. He stated that he didn't want waste 155 gallons of water. While I can appreciate that, who would move a tank that size without water testing it before buying it or before bringing it into the house?
Not your fault Joe. On this site and ESPECIALLY on Craigslist EVERYONE knows that it is buyer beware. People are nervous buying things on Craiglist for good reason and they should exercise more caution when purchasing anything. There was no warranty offered or implied but if you're dealing with your own personal morals steering you to offer him any money back than that's cool but make sure you're not getting played. Have him send you pictures of the tank from all angels and water on the floor and the wet/dry vac he used and the mountain of towels and blankets he used in the emergency situation he had to clean up.
How do you know he didn't try to drill or or maybe even cut the center braces out for unobstructed lighting and the glass bowed to the point that it began leaking.
Bottom line is if the guy didn't know enough to water test it, then the guy is probably a novice and didn't know to level it. There are so many other variables too. What if he dropped a huge piece of slate, granite or Texas Holey rock and cracked it. What if he chipped a corner when it was being moved.
Put those DT skills in to effect and if the guy is near by, take a look at the tank and examine the situation before you shell out a dime. As much as people on CL have to beware of who they're buying from, sellers have to beware of who they're selling to as well. I'd say my dealings on CL are 99% excellent experiences and the few that are unsavory don't detract from the rest but we definitely need to be aware of them and be prepared to deal with them.