Internalization and value capture

When you first learned about the market price mechanism, you understood that it was a method to "internalize" the social consequences of your actions into private consequences so you would always do the socially optimal thing. This is, by the way, an absolutely ingenious fact – that a free society, in some mathematically elegant way, actually leads to "socially optimal" (from some perspective) solutions.

But then the thought occurs to you – how can the reward for your actions (the price of a good you're selling) be /equal/ to the value you created? (Why would you be allowed to keep /all/ the value?) 

Indeed, it is not – from the perspective of the buyer, the price he's buying the good for represents the /cost/ that his consumption creates to everyone else, i.e. the opportunity cost of that resource not being allocated to the /next best/ use of that resource. If you're selling 5kg of steel to a buyer, then the amount you earn is not the value of the 5kg steel to the buyer – the amount you earn is the /marginal benefit/ of the /last/ infinitesimal bit of steel, multiplied by 5kg.