Don't want to say they all are, but...
I bought a used bassinet for my son to sleep in just for the first little while since we had no money and it was small enough to fit in our RV bedroom.
I bought a new mattress for him and it worked just fine. I think it depends on the design and condition as to whether it's really safe or not.
If you can afford to buy new so you can get the recall card with it, you are probably better off though.