New 55 Gallon tank

Ack, some people are too lucky. A new 55g tank would run you close to $100 itself. Add the stand and hood and you are one lucky person. Is the tank currently set up or is it empty and in need of cleaning?

Generally when cleaning a tank a mild bleach solution is the best general use cleaner followed by lots of rinsing. If there are stubborn and noticable water spots vinegar will usually do the trick again followed by lots of rinsing.

The biggest thing I would look for are obvious cracks or chunks in the glass. After that you should take a look at the inside edges to make certain that the silicon hasn't dried out for some reason and cracked. Third you should set the tank on a level surface and fill it with water. If there are any leaks bigger than a very slow trickle you should be able to tell. If the tank is set up and running already then you already know that it should be ok to buy.

The stand should be level and sturdy. If the stand doesn't sit level you can cause stress on the tank when its full which may lead to shattering. The hood should rest on top of the tank and should also sit fairly level.

If it turns out just the tank, stand, and hood are usable, but the lighting and filter aren't it will still be a good buy for $75. Most likely the person is either getting out of the hobby, or doesn't know what he/she is doing and got frustrated.