Net Auction Otoko no Tanoshii Isekai Boueki

Net Auction Otoko no Tanoshii Isekai Boueki (ネトオク男の楽しい異世界貿易) is a light novel series by Kon Hoshizaki with illustrations by Mio Sazanami.

Plot
Ayase Jirou is someone who makes a living by net auctioning things he purchased from people who don't need them anymore, in other words a "Picker." One day, he found a large antique mirror from an old bag's cellar which was given to him for free. That mirror is actually a portal to another world, allowing him to go back and forth between worlds, much like Kakedashi Programmer. In no time he found an exploit that by selling things from modern world to the other world and vice versa he could get filthy rich in no time at all!