Good thing is they add this model store into the FPS Creator x10 so while user creates a level and thinks "hmm a demon model would be great here", he clicks on the library and buys one.
Do you mean it would be good if they did it or it is good because they already have done it? Either way, agreed, that would be the perfect way to market these. Embed them into the tool itself, and you've got the perfect impulse purchase when someone is making a level.