I’ve given away games on reddit in the past. In my experience, this worked best:

• Make sure it’s cleared with the moderators.
• State how long the giveaway will run (often 1-3 days).
• Top-level comments count as entries.
• (Optional) Require commenters to follow some simple instruction, like including the word duck, or stating their favorite game.
• Accounts with less than X karma are ineligible.
• When the time is up, pick a random comment as the winner.

This gives everyone a fair chance against bots, hoarders, and resellers.