Scrapbanking is a way for people to make scrap out of any two craft-able and trade-able weapons. It is better than crafting because the items don't need to be for the same class. If you want a weapon, you can go and buy it and another weapon for a scrap, then trade the other weapon and one you don't want in for a scrap. In that way, you can trade 1 weapon for 1 weapon of your choice.

There are many places you can scrap bank. My personal favorite is scrap.tf.

It provides a reliable way to get your trade your items for scrap or visa versa.

Another way to scrapbank is through a trading site, such as TF2outpost.com. This is a slower method, but can increase your income through people who really want a weaon.