I’m just in the middle of writing an article on the technicalities of the foreign exchange market, and what went wrong, and this example came up. I think the fair solution is pretty intuitive, but maybe others will differ. Presume below that this is a one-time interaction, so nothing to do with reputations, repeat business etc.
“You are on your way to the fruit market, because you want to buy five oranges. Someone you’ve never met before accosts you on your way and says “Hey, you! Could you buy me five oranges please? I’ll give you the money when you come back and I’ll pay you ten pence for doing it”. You think what the hell, and say yes. You ask what’s the maximum he’s prepared to pay for them and he says “Don’t care – whatever the market price is”.
Down at the market, there is one stall which has five oranges for sale at 50p each, and another stall with five oranges for sale but charging 55p each. You buy five oranges from each stall and head back home.
Your customer is waiting back at your gate. He gives you your ten pence, and asks “How much did my oranges cost?” What do you tell him?
You have three choices really (I’d be interested to know if anyone could justify any other price).
a) Tell him “50p each” – ie, you filled his order first and then your own
b) Tell him “55p each” – ie, you bought yours first, and then his
c) Tell him “52 and a half pence” – ie, you give him the weighted average of what you managed to pick up
In case a) your good turn has cost you a pretty penny – you paid £2.75 for your oranges when you could have got them for £2.50, and your 10p wages doesn’t cover the difference. Even in case c) you are down on the deal – paying £2.625 for your oranges, less 10p for an “all in” cost of oranges of £2.525 which is 2.5p more expensive than if you’d never met the guy. A lot of people would say case b) is perfectly fair – this guy clearly doesn’t really care all that much about how much he pays for oranges, or he would have gone to market himself rather than grabbing a complete stranger to do so. It’s also the point at which your profit from the overall transaction (10p) equals the wage that he said he would pay you.
Why should you subsidise him? But on the other hand, isn’t there something a bit hinky about deciding that all the best-priced oranges were for you, and all the worst deals were for your client?
Of course, I think people’s intuitions about fairness might change if your customer was paying you £10 to go to market for him, or if you had explicitly promised him that you would get him the best price possible. But in the simplest case (and this does match up pretty well to the actual structure and pricing of the FX market), I think it’s not obvious at all that the most intuitive concept of fair dealing corresponds at all to the regulatory concept of “duty of best execution”. Anyway, what do you think?
Update the longer post is now up.