They can be. I would be a little wary of any headphones utilising a thin cord with a 1/8 inch plug, reason being thatthey are more likely to short out aftera period of time, resulting in hearing your music out of only one ear...
If the headphones are used, try and find out why they are being sold...Refurbished ones may have had their magnets replaced or their speakers changed out. They also could have a bad cord.
In all honesty, I would probably go with new headphones and try and get something with a warranty. Refurbished stuff is nice if you are just looking for something cheap that if it breaks, you are not out a whole lot of money...I would recommend something along the lines of DJ quality headphones with a 1/4in plug, and justget a 1/4 to 1/8 adapter (stereo of course) so you can listen to your iPod or personal music player...
It's a risk, but depending on the deal it could be worth it.
I would not buy anything of high quality refurbished, like I wouldn't buy $1000 Sennheiser 650's for $800 refurbished.
It's too much of a risk, and there is always a reason behind it being sent back in.
How much are you paying for the refurbished Siberia's?