the piece of corporate finance you are missing is that the money you use to buy the broccoli is scarce, you have to "pay" for it. you can't leave it out of the calculation. You pay for it to who you borrowed it from, or to your shareholders, or the opportunity cost of taking it out of whatever use somebody else was paying you for.

that dollar you use to buy the broccoli, it's like a machine you use to knit sweaters. If you put that dollar to use for a year and sell one broccoli, you made 2%. If you put that same-single-dollar to use for a year and sell 52 broccolis, you made more.

what I originally said was that you can't just look at the margin on a sale, you need to look at inventory turn, how many times you collect the margin.

what makes the margin low is competition. a grocer is willing to accept a 2% margin on a sale because the cycle can be repeated many times in a year. In a sense, the interest rate (which is quoted at annual rates) on the borrowed dollar is fixed at that annual rate, but if you get to cycle through your operating capital quickly, you get to look at it as the amount of interest you need to pay each week.