I too, would be a little cautious of refurbed HUs. My friend bought a Clarion DXZ835mp from Indoaudio refurbed, and it had a problem. Then again, my brother bought a refurbed Pioneer HU from Indoaudio, and it works fine.
Here is my view of refurbished stuff. If it is factory refurbished then i would not be skeptical at all. I own a factory refurbished 300/4 and it has been rock solid i have never had a problem with it. I have had for almost 11 months. If it is not factory refurbished its up to you, but i personally would not buy it.
I stay away from refurbished anything but 100% from anything refurbished on the net. mainly because it's too **** hard to go through the process of getting it replaced. I'll either pay the extra for a new one or downgrade the model to one I can afford before going refurbished but that's just my personal preference.
refurb isnt a bad idea, but a HU is the thing that gets the most wear and tear, you are touching it, its got moving parts, cd players are very finnicky, bla bla bla.
refurb\used on the sub- thats fine
refurb\used on speaks- cool
refurb\used on amp(s)-sure...
wire? lol refurbed wire?- ok...
refurb\used Head unit?- no, this neeeeeds the warranty. these things break way too often not to have a warranty. so get one with a warranty (NOT ON EBAY) and be glad you spent a few extra for it. its worth it.