A non-dead one is the main thing. The power supply brick is a separate piece... that often gets separated. I've bought two that were "untested" due to no power supply, and they worked fine once reunited with one. Also, sometimes a "dead" one just needs a new power supply (which are available in various forms).
If you're after a Cube for aesthetics, then the plastic is the part that usually gets scratched or cracked. The computer hangs from inside the plastic on two screws, so look for cracks there. Ask the shipper to generously fill the empty space under the computer with padding and ship it upside-down to reduce the load on the two screws.
The Cube is a great computer, good luck!