From TF2 Wiki
What is this?
Scrap banking is a metal making technique used by players. It is commonly used and can be used on trade servers within the game.
How do you do this?
In crafting, you can only combine certian weapons to create scrap metal. Like smelting class weapons, only some players do not have weapons of the same class in order to craft scrap metal. So a scrap banker's job is to give 1 scrap metal for any 2 weapons traded. So it is like crafting weapons of different classes, only in trading. So everybody is happy.
How can you make profit?
Scrap banking is popular in most trade servers, so many players look for scrap bankers. You will get enough weapons from people to make your own scrap metal from what is givin to you in a matter of trades. Eventually your scrap doubles, but you must start with a good amount of scrap just incase players do not give weapons you are able to craft.