Junk is a way of getting around the GE's flawed pricing system. So, in this instance, people are willing to buy these leaf-bladed swords for far more than the GE allows them to. The seller puts the sword, and some 'junk' in the trade window, inflating the price, and then the buyer matches this artificially inflated price, getting the sword, and some useless, worthless 'junk' in return. So, in reality, the buyer bought the sword for 1m or something. While he also got 'junk', nobody buys this, so it's pretty much worthless. Anyways, fletching is always a good way to get some extra levels and junk as well.