[Cpl. Shrike]

Well that depends.
If the mods show up in the steam games list under "MY GAMES"

You should be able to start a listenserver.
But be sure to have NO network connection on the pc.

If there's no network connection steam goes into the OFF line mode.

Once steam detects a network connection it'll try to communicate with the master servers. In that case you can not start a server.