You buy the junk for trade price, then you use that junk plus items to upgrade or downgrade items or for item lending (Fletching,summoning,willows) are decent junk to obtain
JUNK: Unusable items, nobody wants them ( bows(u), scrolls etc), they are used to trade money,
EX
you pay 3 usd for a million, so u trade him 1m junk, and he give you 1m cash