Net Auction Otoko no Tanoshii Isekai Boueki

Ayase Jirou earns money by reselling items bought from people who no longer need them through online auctions. After receiving a large antique mirror for free from an old man's cellar, he discovers it serves as a gateway between worlds. He quickly realizes he can transport goods between the two realms and uses this to his advantage by trading items back and forth, generating significant profit.