The first thing that comes to mind is glass heaters for me. Unless they are very new, I would stay away from them.
I would shy away from used MH/T-5/VHO lamps as they have bad color shifts as they age. Fixtures however are fine, just assume the lamps are to be replaced or buy without the lamps.
Live rock is questionable as it can always be cooked or acid bathed and re-cycled. I would hate to inherit another person's nuisance algae or coral eating pest, however if it does have pests and algae it is usually dirt cheap and can be re-cycled in the long run...
Anything electronic with a stiff power cord. This is usually the sign that the cord is ready for imminent failure. If it fails though, you are playing with a good conductor and can cause fire or death easily. Make sure the cords are good.
UV-Sterilizers - make sure the age on the lamp is verified or assume it needs a replacement.
To be 100% honest, the only BRAND NEW piece of equipment in my tank are my dosers. Everything else I have purchased used either local or online through R2R... Even my refractometer was bought used
I am very budget minded and I tend to let my corals support the expenses of the tank and hobby. I also shop sales well and do lots of DIY to keep things on the budget.
