I buy everything used.
It just makes sense.
Yes... and no...
It's a hobby. One I've been at for a long time. I figured out long ago that if I buy something that is less than what I really wanted, I'll just end up buying what I really wanted later. Buying twice is no way to save money.
Take that big glass box, for instance. My current system, I invisioned a fat, wide, short aquarium for a specific location and a particular 'look' I was going for. Good luck finding a specific, odd shaped tank in perfect condition, used. Had to go new to get what I want. DBP makes an 80 gallon rimless tank, 48x24x18. It's close... I really wanted a bit longer, a bit wider, but it would have worked. I ended up ordering a custom 60" x 30" x 18" tank from Planet Aquarium. Could have bought several of the DBP 80g frag tanks for less money. Why did I go that route? That's what I wanted. With a little luck, I'm going to be looking at that glass box for the next decade. If I'd have gone cheap, I'd be kicking myself for a long, long time. As to cost, well, if you really think about it honestly, the glass box, even a custom glass box, does not make up an overwhelmingly large percentage of the total cost of a reef system. Add up all the other major components, I bet most of us would have several times as much in other equipment than we do in that glass box.
Sometimes, it pays to get what you want. Even if what you want is custom, and expensive.
Now... I did light that expensive glass box with a used Tek T5 system

Why? Why not? It was cheap, and IMHO, there's nothing better than T5's for lighting a reef. Put in new ATI ballasts and bulbs, just like new.