The biggest thing to look at when buying a used tank is ask how old it is and how long it has been sitting empty. I bought a 55g that was 15years old and was filled everyday till I bought it. And 150g that was 6 years old and had been empty for 1year. The seals on the 55g where great and never leaked. The 150 however had multiple leaks and was a nightmare to fix.
So make yourself knowledgeable when buying it. Of course my occurrence isn't always normal but I would ask the seller to have the tank filled to the max before you come and look. If its not leaking it only takes 5mins to empty a 100g tank so it's not that big of a deal to take it home if it's good.
Also if there is a small leak you could negotiate the price down and fix it yourself.
I didn't do this on the 150g. Hence my advice to you to save you the trouble and money.