We have considered a market place, and generally like the idea. If we did open such a feature though (and there's a LOT to do before we start this!) we would make sure it only listed high quality resources.
There are some resources that are very difficult for indie devs to get hold of, for example high quality sounds and music. I think those would have legitimate benefit for a lot of game makers and can definitely see the value in that.
Sprites are always hard. To get enough sprites for people to buy and make lots of different types of games means you need a lot of sprites. Sprites are incredibly time consuming and expensive to make if you want to hit an excellent level of quality. So that would be quite difficult.