"Large supermarket chain" also provides the trade functions "assortment of goods" and "transport", so what exactly are you paying your extra 50c for? Liquidity.
What a bunch of assholes those 7-11 types are. Charging us an extra 50c for milk. The nerve.
However, I'm afraid your proof-by-analogy does not work. If you actually know what you are talking about, why not explain the current financial system clearer and with more information?
In turn I promise not to burden this discussion with a dissection of your analogy, or even my own set of analogies which would beautifully fit my current bias, and which you would surely reject outright.
Market makers generally, smooth these time disparities. As long as the smoothing is cheaper than the price disparity then it is a net positive for the "real" participants. How do we determine if this is cheaper? Quite simply, the market! You can trade non-liquid products electronically. The price spread his much higher in those products.