Elf Deck To Senjou Gurashi
Summary
Our unnamed protagonist is an ordinary high school boy who loves Magic Walker - a super-popular card game. In particular, he loves the Elf Deck: a selection of cards that, while seemingly weak, can be incredibly powerful when played right.
One day, while on his way to get a new elf card, he gets bullied by his peers. One thing leads to another, and soon he wakes up in another world in the body of the elf character he was hoping to get his hands on. And wouldnt you know it, he can summon elf cards from his old world too.
In a system like that there would need to be rigorous documentation and certificates of prior ownership/guardianship or else anyone and everyone would just get grabbed off the street and enslaved at any time, and such a society would collapse instantly. So these slavers wouldn't have such documentation unless it was a forgery since the kids were just kidnapped; both offenses should be very serious capitol crimes. Once again, the day is saved by glorious Bureaucracy! (and the MC's knowledge and exploitation there of).