Many games with an online component, allow players to buy various kinds of content. As far as I know, none of them allow a secondary market where players resell purchased content to other players. Presumably this is because the developers think disallowing the secondary market will get them more sales.
However, there seems to be a widely held school of thought regarding Magic: the Gathering (the nonelectronic card game) that the existence of a secondary market actually increases revenue by encouraging people to buy more cards in the hope of being able to resell them. Whether that theory is true or false, the mere fact that many people believe it, should encourage at least some video game developers to try allowing a secondary market.
Given the existence of a reasonably widely held theory that this is a good idea, why is it that no video game developers whatsoever allow a secondary market?