I Got a Cheat Skill in Another World and Became Unrivaled in the Real World, Too, Vol. 1 (light novel) - Kuwashima Miku

A Chance to Come Back! A mysterious door stands open, inviting a boy who’s been brutally bullied all his life to take a courageous step forward into the unknown. On the other side, he finds a hoard of priceless artifacts and a world as filled with magic as it is with monsters. The most shocking revelation, however, is that he can bring whatever he wants back with him when he returns to Earth. It won’t be long before this double life changes him forever...