Netooku Otoko no Tanoshii Isekai Boueki (LN)

Netooku Otoko no Tanoshii Isekai Boueki (LN) summary:

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 Kakedas.h.i.+ 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!