It may turn out to be a good project at the long run but their is a high probability of people to only purchase the cards only at festival periods, don't you intend selling on normal days? On the issue of private keys; Nobody will be quite confident to trust anyone to give out a thing like that, even if you mean well for them. And for selling offline, well this might work for those who may not trust you for offline purchase but know that this will limit your business and also most bitcoin owners or holders prefer been anonymous coupled with the recent news on bitcoin stealing at gunpoint.