Scrap metal was meant to only have value when traded for other cards on the market, to urge the player to try trading at least once, and also to have the chance to use the market without losing something you might want to keep. I am not sure if you ran into a bug with the market, because they shouldn't return to your hand if you trade them. If that's the case, it's a fault on my part!
I tried to design the game to be intuitive enough that players would figure out the mechanics with minimal guidance, so it is totally understandable if you ran into a point of confusion. You comment helps me a lot when trying to figure out how to do it better.
1400 is a good score! Thanks so much for playing and for the well thought out comment!