It's a cool idea and I like the design, however there's a major flaw in your system.
In my opinion, the 6 BTC reward is what will draw some customers in. The fact that one customer can then post the codes online for everyone to see isn't really fair to the other 19 coin owners as they'd have had to pay to get the coins containing the codes and now their chances of getting the 6 BTC have diminished significantly. If I were you, I'd try finding a way of stopping gamut Hong like that from happening.
Not sure on what you could do about it though...
In my opinion, the 6 BTC reward is what will draw some customers in. The fact that one customer can then post the codes online for everyone to see isn't really fair to the other 19 coin owners as they'd have had to pay to get the coins containing the codes and now their chances of getting the 6 BTC have diminished significantly. If I were you, I'd try finding a way of stopping gamut Hong like that from happening.
Not sure on what you could do about it though...
I agree with the other 2. People who buy this wouldn't do that because basically they would devalue their coin. If someone did I would not sell to them again obviously.
But after the the prizes are claimed I do expect the back of the coin to be public to prove that it was all legitimate and the 6 BTC was claimed and the private keys did contain the prizes stated. Well public as far as 90% of the codes on there. There is 10% of the code that leads to the private key of 10 BTC prize.
I just can not see anyone posting this online unless someone wants to jeopardise the project. So if anyone does they will be banned from further sales. I dont know anyone who would give away a treasure map knowing it would make them less likely to find the treasure and also devalue the value of the map itself.
I think its a good point worth mentioning but it wont happen and if it does the source of the image online will be banned from any other sales.