First of all, Shop Titans is a game that changes over time: The low tier items you make when you start are made in a very short time. And, as Rugus remarked, you can more easily sell them to higher level players who can't be bothered making this low level stuff rather than to NPCs. This changes at higher levels: Items of normal quality at higher tiers sell for less than twice their base level on the market, so selling them with the surcharge function for twice their base level to NPCs becomes the preferred mode. Which leads to some sort of dance in which you sell lower tier items at a discount to gain energy, and then use the energy to sell higher tier items at surcharge.

But more importantly, higher tier items take over 1 hour to craft. Which means that you aren't actually "playing" Shop Titans for more than a few minutes at a time. This is a very different mode of gaming, compared to playing some action game on console or PC. But on mobile there are a lot of games like this, which are designed to be played X times per day for Y minutes at a time. You need to be rather patient, as progress with this mode of gaming is obviously very slow, over weeks and months. Each individual play session is rarely exciting, but there is a sense of overall progress with time.

I tend to play always at least 1, but not more than 2 games of this kind on my iPad. Currently that would be Shop Titans and Assassin's Creed Rebellion, with the latter getting less interesting, as I am at the level cap. In Shop Titans I am still far from the cap, and it will take me many months still to get there. And as I mentioned in the previous post, I am still slowly gaining understanding of some of the game systems and getting better at it.