1) How do you guarantee the holder of the card that the coins are still available? Currently, we have to trust you (the emitter) *and* the fact that the sticker has not been removed, which looks a bit gross (it shouldn't be that hard to replace a sticker with another one).
Well, you can always look the block chain too. If the money remains in the address, you should see at block explorer for example. But yeah, on offline transactions, you have to trust it's not a fake card.
2) How could the user be sure that the card was emitted by you? I mean, I could print a bunch of cards with QR code and stickers, sell them for 1 or 5btc. The holder will exchange them and a the time someone try to cash them, nobody will be able to trace the card to me.
I think he expects the holograms to be enough for it. And, well, it's pretty much the same thing for state money bills...