When you purchase a used tank, consider the following -
Is the tank and other equipment it good shape? If it's not close to perfect, what will you need to do to correct this? Scratches in glass can not easily be removed. Broken or damaged tank frames are a lot of work to repair.
Does the tank fit into what you want? For example, if you wanted a 125 gal tank, and a 75 is available, it would be a step downs for you.
What are you going to need to spend to get the additional tank equipment you'll need? The filtration system, a skimmer, and lighting can cost way more than the tank. After all is said and done, you may not be saving very much.
What is going to be the TCO, Total Cost of Ownership? Don't forget things like the amount of power it will take to run it, filter media replacement, bulb replacement and so on. You might find a 75 goes way over your budget or you might find that a 125 will not cost that much more. A lot depends on how you feel about the system and what you want out of it.