How do we get game trading?

Steam is evolving gaming and making changes that will effect our favorite pastime. Greenlight is bringing more varied and experimental content to the market cutting out the marketing guru's that have squashed so many projects. Early Access further supports studios by allowing them to tweak and refine projects before release, improving quality and playability, how many great games have been passed over because minor glitches or unrefined controls! Trading Cards now provide financial rewards as well as achievments. This is all great stuff but how many completed/unused games are in your library, if we were able to trade those games it could possibly detract from revenues reducing the purchase of new/sale games, although family sharing could potentialy have a similar effect. Equally it could further promote individual titles increasing their fanbase. So how can game trading be managed to be good for both gamers and developers? Can Steam fix the price on trades passing on a cut to developers? Should game trades be limited to titles where sales have hit a certain level? What do you think?