First,
No you won't be able to.
Second,
SINS has a CD key that gets tied to an account when first installed. If you buy a second hand copy you will only be able to get patched through 1.5.
Stardock will not host multiplayer for second hand purchases.
You can however, purchase just a CD key for your used copy and recive all the patches and have access to multiplayer.
I would strongly recomend purchasing retail and save yourself the trouble of buying used and then buying a key.