Buying stake in a company and purchasing a commodity are different things. If I buy stock with my wages, I might consider that stock capital because it will provide me income based on the labour of employees of that company and not my labor. And there is an argument that arbitrage of stock is capital accumulation.

But if I buy a commodity and resell it, nowhere am I siphoning someone else's labour to make that profit. Nobody is working for me to increase the value of the item I possess. My repricing and reselling might even be considered my own labour. I paid market value for a commodity and am reselling at a higher price. It may be unethical, but the gain in money from the arbitrage of goods is outside of the cycle of capital accumulation. Whose labour am I stealing value from?

Edit: I might be changing my mind about my definition of capital