@VladV:
I am not sure about that.
E.g. a lottery machine with 1 golden ball, 2 silver balls and only red balls otherwise that the participants spin and then hope for the golden ball to drop out. (That is basically the same as a gacha.)
The golden ball is the jackpot, the 2 silver balls are the 2nd ranked prizes and all red balls are consolation prizes.
If you make such a lottery and someone gets the golden ball or one of the 2 silver balls you then have to mark on the winning sheet informing the participants before they spin the lottery machine that the prize for that ball is taken already.
Once all winning prizes are taken you would have to make that obvious to all those that would still come for the lottery.
After all you would otherwise suggest that they can win the prizes even though they are already taken and there is no chance to get them anymore.
A gacha machine is basically the same thing.
If certain items are completely taken you must make that visible to anyone wanting to try the gacha.
If they do not have to do that then they can simply cheat like this:
1. Place the 1 rare item so that it will be taken with the 1st draw - I am pretty sure you can just place it in the mechanism that chooses the item to be handed out.
2. Have someone you know come "by chance" to draw the gacha directly after you restocked the gacha machine.
3. That person draws the 1 rare item "by chance".
4. You let people play the gacha knowing that there is no chance for them.
5. Once it nears becoming empty that trusted first "customer" returns the rare drop to you.
6. Then you fill up the gacha machine the next day and it all repeats.
==> The shopkeepers could cheat that way without anyone ever being able to prove it.
Therefore the only way to avoid such fraud is if the shopkeepers would be forced by law to mark the machine as not having the rare prizes anymore if they have already been drawn.