When you are making premium / innovative products, an upfront investment needs to be made. Since the denominator in ROI is high, the ROI remains low. But you can charge a premium for your products and your net profit margin (Net / Sales) can be high.
When you are trading goods (buying from one marking it up and selling it the other), there is no investment involved. So, ROI is high. But you are not adding any value to the product. Your sales are high but since there is no value addition, you cannot charge a premium and the profit margin is lower.