Used can be such a pain though. Don't make me show you pictures of tanks that have ruptured, cracked, or leaked on me. Many people just don't know how to move and setup tanks properly. Ask the person a few questions before you buy to be sure that he is not a complete novice. See how the tank is currently sitting, whether it is still running or not. Probe them asking questions that may show that he/she has broken one of the cardinal rules:
1. No glass trank, 5 gallons or more, should
ever be moved with any water or gravel in it, period. They were never designed to support the shear stress that came come from uneven supporting. The tank may look fine but the seals may be seriously weakened or have come loose from the glass. The same goes with acylic but it usually will support far greater stresses without damage. Problems may not come for weeks or months later, or when you accidently bump it in the middle of the night
2. Nearly any tank (especially glass) over 75 gallons should have an aquarium pad directly underneath the tank
and stand. Carpet may be substituted for the stand's pad. The only exception would be a perfectly smooth and verified level floor but the tank must still have a pad. This becomes even more critical as the tank gets taller and/or longer (ie. 80 & 110 tall, 100 & 125 long).
3: No crack, whether leaking or not, is
ever ok. There are people who can fix this and still have it safe for fish use, but it's usually rare, expensive and still not a good idea. Multiple times I have heard someone speak of inserting or removing liverock or decore and hitting the glass causeing a small chip or crack that doesn't leak. I have seen the results of such a small crack when a father tossed a softball to his toddler son and it hardly tapped the front glass, shattering the right side (with the crack) of a custom 300 bowfront. Just stop and picture that for a moment before buying a used tank. He might have saved a few hundred on the tank, but most likely cost him everything in his home (unbelievably, he is back into aquariums again).
I'm not trying to discourage you from buying used, only that you use caution. In fact, all but one tank I've had in the last two years were used. If there is any doubt
, WALK AWAY!