If a ten year old bike was made of steel with no rust, I would have no worries. However, I've always thought that an aluminum bike over ten years old, most likely has been ridden quite a bit. Since aluminum has fatigue issues, I'd be hesitant about purchasing an old aluminum bike.

Components can be swapped. It's not so easy to swap frames...

* If the aluminum bike hasn't been ridden that much, it is indeed of greater value.