A member doing supply runs is being paid for delivery. A supply business is paid for the goods. The distance between them is mostly capital and confidence, and the knowledge required is built by doing the runs.
What the route already taught you
Which shops buy what. How often. What they pay. Which suppliers are reliable and which are not. That information is the substance of a trading business, and most people who want to start one do not have it.
What actually changes
You buy before you sell. Instead of purchasing on a merchant's instruction, you hold stock and sell it on. That is working capital tied up and at risk.
You take the price risk. If prices fall or goods do not move, the loss is yours. As a runner it was never your exposure.
You earn the margin, not the fee. Which is the whole point, and it is usually the larger number.
Start narrow
One product line you understand, for shops you already serve. Buying broadly at the start is how most first attempts fail: capital spread across goods you do not know, sold to shops that had not committed.
Credit is the trap
Shops will ask to pay later. Some will be good for it. A new supply business with all its capital sitting in other people's shelves cannot buy next week's stock — and that, rather than lack of demand, is what usually ends these businesses. Decide your credit rules before the first request, not during it.
Keep the runs going at first
Supply runs pay immediately and cost you no capital. Running both while the trading side proves itself is how you avoid betting everything on an untested assumption.
Becoming a merchant
At some point this is a business rather than member work, and it can be registered as a merchant on Knomart with its own records, stock and customers. That is the loop closing: the member who used to look for work becomes the business that posts it.