It was about this guy that has some power or item that is used as a portal to another world. There, he worked as a merchant and get profit by selling the gold in japan. Thank you (Sorry for bad english)
Netooku Otoko no Tanoshii Isekai Boueki There are several novel like this but I think this is one you looking for. I only skimmed through it but here's some information. -The MC was given antique mirror that happen to be portal to another world. -The MC sell a knife and got a lot of gold coins because stainless steel is beyond that world's technology. -The MC then sell gold coins in Japan.
Yup that's the one, I didn't remember the spesific, just remember he make load of money from selling the gold Thank you