Junk trading allows buyers to buy something for cheaper than what the GE states its value at. For example, someone could buy a Top Hat for 3m + junk. Meaning they are essentially paying 3m for something valued at 4.5m, but they need to cover the 1.5m with something so that it can be traded. This is done with junk. Partyhats, Top Hats, and a few other items are traded this way. It also allows sellers to sell something for far greater than what the GE states its value at. This is done for items which are rarely traded on the GE because they are in fact so rare. These would be things like animal masks, canes, leaf bladed swords, and all 3rd age armor. The seller will add junk to their item, increasing the value at which it is to be traded. Common junk items used in trades which have no realy value, but have a price on the grand exchange are onyx necklaces, summoning scrolls and pouches, unstrung bows, and gold jewelry.