Avoid G2A or Kinguin (grey markets). Use:

For players looking for enhanced multiplayer features and better security against legacy exploits, the community often uses: Call of Duty®: Black Ops II on Steam

While the nostalgia is powerful, the risk-to-reward ratio is catastrophic. You are gambling your personal data, PC security, and legal standing to save $20 on a 14-year-old game.

Even with a legitimate copy, the original PC multiplayer servers for Black Ops 2