Well, the ad doesn't mention rust or leaks at all but yeah those would be bad. They still wouldn't necessarily be a deal breaker, though. The guy is posting it for $250, you could probably get it for $200. If you show up and the tank is fucked, offer him $150-175. He seems to just want to get rid of it. Just compare it to the rest of LA and OC Craigslist prices. You won't find a roller for that price, let alone a whole bike.