The shop selling/buying has some formulas, but we should only care about "Misc" items. For them:
+ The buying price is double the selling price.
+ The selling/buying price increase 2.5 times per rank.
So to put it simple, let say you can buy a rank C item from the shop for 100 SC, here is the (theoretically) approximate value of what you get when selling/buying similar items with different rank.

Crafting "Misc" items requires two items of lower rank to get one item of higher rank, so the theoretical value of an item is doubled per rank. However, as mentioned, the selling/buying price increase 2.5 times per rank. Therefore, we can conclude:
+ Selling two items of the same rank yields less profit than selling one of that item with 1 rank higher,
+ If we gather enough low rank items and craft them to higher rank ones to sell, we can make profit.
(Math) Assume we need to level up a bunch of items at least n rank before we can make profit. Then n must satisfy:
(2.5^n)/2 ≥ 2^n
Therefore, n ≥ 4.
So in order to make profit, we must buy (a multiple of) 16 (2^4) "Misc" items (preferable rank C) and rank them up (to preferably rank SS) and sell them to get profit.

For this method to work, you will need access to survival mode as crafting "Misc" items from rank C to rank SS is required. As you can see, I have a Bones crafting set from B to SS.
Playing in "Insane" mode is much more preferable because for most misc items, you can get recipe and crafting recipe for rank C and B from playing story mode. Also, you should get at least 32 bag slots to reduce the time switching between tabs.

Now that you have read through the mechanic and requirement, you should have the idea of what to do.
1. Get a crafting recipe of a "Misc" items from B to SS. (Weapons and Accessories require a complex combination of items of rank up, while "Misc" items are much more simple)
2. Disable all crafting, selling and buying confirmation. (Important, as enabling that will cause you to lose twice the time.)
3. Buy more bag and sell items in your inventory to make room, as much room as possible. You can buy back your sold items later as long as you don't exit with the same price.
4. Buy a multiple of 16 items from the shop. As you can see from the image below, rank-C Bones cost 205 SC a piece, so 16 of them cost 3280 SC.

5. Craft the rank-C items to B, A, S then SS and sell them. For 16 rank-C Bones of total 3280 SC, I crafted a rank-S bones with 4000 SC, making a 720 SC profit.

6. Repeat 4-5.
7. Remember to buy back your items before exiting.

Comparing to farming the King on Level 12-Stage 1, this method may seem more time-consuming. However, the advantage of this method is that it follows a strict path with known result and you do not have to switch or change screen a lot, which should save a lot of time. Hence, you can abuse this method by using Auto-Clicking Macro Software, and able to make 150000++ SC per hour. Of course, Cheat Engine will do that just fine, but it has the word "Cheat" in its name. Plus, using Auto-Clicker means that you have fully understand a game's mechanic, which is different cheating. Anyway, the choice is yours. Have fund making SC.