Expand your store to sell more each tick. Of course you have to pay more maintenance on the store. So you need it to have full shelves not empty shelves or it's just a waste of cash. So what I've started doing, is making sure everything I'm selling will be in stock for ~26 hours and then spending any remaining cash on expanding the store.
Lazygun Ltd started out in the lime business, branched out into tomatoes and apples (because apples were at that time only 80% demand fulfilled), got into fruit concentrate and beverage production, reasoning that with a glut of fruit production, they'll be cheap raw ingredients. That's when I realized I couldn't buy paper cartons anywhere on the market so I had to set up a tiny little paper factory which nevertheless produces far more than I can use for myself so now I may start diversifying into the paper business.
Meanwhile I started a small farmer's market which I'm now keeping stocked and slowly expanding. I reckon if you want to find a profitable industry you can build a small shop, purchase small quantities of everything it sells, see what sells well and buy more of it. If there's something you can't buy or can't profitably buy you might have found a gap in the market. Unless the reason is that some megacorp has tight vertical control of the whole market so it's saturated even with nothing showing up on the B2B.
The first night I did make sure I had enough cash in reserve to pay my interest bill. But I got hit by wages and building maintenance for 200K so was forced to take out an extra 80K loan.