Be aware that, other than the Shearwater brand, I think every computer that you could buy used will NOT have a warranty. I.e. I think Suunto (and everyone else except Shearwater) only gives a warranty to the original owner. So, if you buy a used Suunto and it breaks on your first dive, you are just out the money.
When you don't know enough about them to know how to thoroughly check it our yourself, you might be better off to buy something brand new.
Also, it can't hurt to read this:
So you want to buy a new computer?