I agree with Hamperking68, they get more expensive because there are less of them now than there was when they were new... And some sellers, as mentioned above, believe that their $35 motherboard is worth $100, and will then sell it for $120 and wonder why nobody buys it.
Law of supply and demand: If supply is higher than demand, the price goes down. If demand is higher than supply, the price goes up. Eventually, they even out into a Market Clearing Price, but that's a lesson all in itself...