Even something as small as this would help. Trade-ins will get you "Steam credits" which can be used for other games.
A game that's less than a week old you'll get to trade-in for 33% the value
A game that's less than a month old you get to trade in for 25%
A game that's more than a year old you get to trade in for 15%
This would benefit both steam and the customer, steam would save on some bandwith and the customer would have that extra few credits to buy a new game. It's not like retail where a second hand copy goes from person to person.


