He could start industries, patent technology. But he still just sells matches and soap..... The plot doesn't make sense.
In addition to the issues that others have pointed out, there's a lot to setting up industrialized manufacturing beyond having just a bare-bones understanding of the process—even leaving aside the fact that the bare-bones understanding he could get from YouTube and Wikipedia likely wouldn't be sufficient—For example, logistics is another major factor and something he's unlikely to have much skill in.
There's also the issue of all the modern, technological, components that modern industrial process rely on. For example, any process that relies on automation (almost all of them) or precise measurements (many of them) is going to require tools that incorporate a lot of electronics and integrated circuits, which he'd have to buy from the modern world. Sure, he could theoretically amass enough wealth in the fantasy world to pay for them but A) that's going to take a long time and B) by the time he accomplishes that he'll already be making enough money via other methods that he wouldn't need to go into manufacturing any more.
Man, he can convert money. You can get capital in a lot of ways. In Rise of the Bookworm, for example, she made paper in a year (handmade from scratch) and the merchant invested in the production. And here he can just bring in the finished product. Why would he go into the trade in the first place? There are far more valuable commodities than matches. And that's not to mention medicine or any revolutionary discoveries.
Some of those things
might be worth more in the fantasy world, but they also cost a lot more in the modern world (or they take a lot more effort to get up and running). The matches and other cheap goods he's, effectively, drop shipping cost him very little but earn him a lot, so he already has really good margins and it's questionable that he'd actually get better margins with the more expensive goods. (The way people value things is linear and they don't have unlimited disposable income to throw at him, meaning the costs of more valuable goods from the modern world are likely to grow faster than the profits from selling them, shrinking his margins in the process).
I'd also question the idea that modern medicine, especially anything that's over the counter, actually beats what the fantasy world has. Sure, they don't have modern medicine, but they do have literal healing magic and potions. They also have a maladies and some of them have drastically different biologies, so it's not a given that the stuff he imports could even help.
given how much modern day 'average' ppl spend on 'skincare' products i bet he'd make a killing selling moisturizer to nobles lol. maybe he can get his grandma to 'enhance' the average japanese knicknacks he'd buy from a convenience store or so to make it even more effective/valuable
Keep in mind that part of the reason people spend so much on beauty products is because of marketing and cultural expectations that say they should, as well as beauty standards that are based on people using those products. That may be less of a thing in a historic setting, even if they still value the results those products produce.
(what if) Plot twist Aina actually the princess 🗿
Do we know the timeline for when her father died or was drafted? Her mentioning that he had a similar bracelet to the Owl's + Owl's father saying his son should keep any promise he makes with his chosen business partner, even if it costs his life, makes me wonder if the actual twist will be that Aina's father and the Owl's father were partners and their disappearances are related.
other than making a profit and just wanting to work with this capital's guilds and stuff it's not as if he's aiming to be like 'world's best merchant' right? I assume he still has a lot of money from the festival or so and is just working because he has the chance/opportunity versus just lazing around/enjoying some 'luxuries' til he needs to restock again lol but it would be cool to see him introduce some 'new' stuff that hasn't been sold yet (he should go to those candy stores where they sell it by weight and resell the individual ones for like 4x the profit 8D)
He's mentioned that his goal is to make enough to retire comfortably, for the rest of his life, and he's well on his way to accomplishing it. Similarly, we've already seen him expanding his stock (pretty significantly). Also, he's consistently, and intentionally, chosen to not overcharge
too much for his goods, so I doubt he'd want to start ripping off literal children (who probably don't have that much disposable income anyways).
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