A used tank is a perfectly good option. Others may have additional specifics to add, but here is what I'd look for:
1) depending on the size of the tank, is the center support across the top in good shape (this is CRITICAL)
2) aesthetics - is the glass or acrylic scratched?
3) Wear and tear - is the trim notched or chipped from rough handling? Missing trim? Any cracks in the glass (duh).
4) Sealant - If it's soft and seems intact, it's probably ok. However, set the tank up outside (make certain the surface or stand is perfectly flat!) and fill 1/3 to 1/2 with water. Wait and watch 24 hours. Fill another 1/2 to 1/3, repeat cycle until tank is full, observing for leaks. If it doesn't leak, you should be ok. Just because it does leak, does not mean you can't reseal it!
5) if you are paranoid, or particularly cautious, you can always re-seal the tank. It's tedious, but not hard and the proper caulk is inexpensive. There are several threads on this forum that discuss resealing tanks.
Saving $$ is always a good idea!