As far as site and trading rules go, I feel like a "mystery box" would be against trade rules as it doesn't specify what is actually being sold and/or raffled off.
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Be specific in your trades. List everything that is for sale, especially in packages, with as much detail as possible. Unlimited quantity sales are not allowed.
Like VentiRAT said, you open yourself up to scammer accusations. Specifically from users that may feel like your lottery was staged, or misrepresented. You could essentially charge users 5fg for entry, and be raffling off something completely useless or not even worth the entry fee.
If you want an honest answer from a staff member, I would recommend sending a PM to one with your questions in detail of how you plan to execute something like this and get an honest and informative take on it.