coming to this thread late (and honestly, just haven't had a lot of time to spend on the forum lately, sorry about that), but this is truly the nature of the beast.
we in the hobby take risks day in and day out. you can buy coral from a reputable seller / bro, dip it, and STILL get bugs. There is no guarantee - there's only reasonable assurance. When you are buying NEW things, the motto is "buyer beware", this is doubly true when it's used goods.
let's be realistic here for a moment - as others have pointed out, this is a relatively small community, it's not craigslist. you don't TRY to sell a knowingly leaking tank, and if you do, you would be upfront about it and let the other party know that they need to reseal it. And say the buyer and seller DID do a water test, then what? it was already noted that it was a slow leak. in the process of filling it up, would it really have shown the leak? i'm guessing the buyer and seller weren't going to stand around for a few hours to wait.
I don't believe the seller knowingly sold a leaking tank. but that's neither here nor there. the transaction happened, there was no stated nor implied guarantees, not to mention that silicon is literally just glue, and susceptible to temperature changes. we have no idea how long it was in transit, did it sit in the trunk for a period of time in the heat? did it get left out in the backyard to get washed and waited to be dried? did it get bumped during the ride? those are all legitimate things which could have caused the leak - the bottom line is that assigning blame is really just trying to close the gate after the horse has left the barn - it's not going to do anyone any good.
I would suggest the seller refund the buyer 30 bucks, which is more than enough to buy razors to scrape off the existing silicone, buy new black silicon from HD, and resealed the tank. I did it on my 180g, and it cost me about 14 bucks (i already had the silicon gun though).
no one is going to "win" from this, but let's stop focusing on what WON'T solve the problem, and try to work toward something that will.