As someone whose been selling pc parts for quite some time, this is not surprising.
Most computer parts tend to follow an unusual trend, and the price trend varies by part:
1-2 years old: prices go down as demand for older RAM reduces.
5-7 years old: Prices go up as people try to upgrade their older PC
7-15 years old: prices crash and approach scrap value. This has currently happened to DDR2. I cannot make money with DDR2 because I buy it for about $8 scrap and can only sell the same memory kit for $10. Exceptions exist like special high capacity modules (4GB sticks) or branded memory (think Corsair).
20+ years: at this point something interesting happens. The part becomes known as “vintage” and starts to go up again, but only the highest capacity sticks available for that particular platform. As an example, 4GB kits of DDR1 are worth about $30, but 2GB kits of DDR1 are still only worth their scrap value.
1-20 years: CPUs have an almost linear trend of depreciation. Some CPUs (like i7s) lose value more slowly than others, but in general CPUs just go down in price.
20+ years: The whole “vintage” thing happens again. Top-of-the-line CPUs for a particular socket (think slot 1 or socket 7) shoot up in value, while older CPUs essentially become scrap.
Motherboards in general actually do a very good job of retaining value. I think this is because motherboards are usually the first thing to die when a system is upgraded, so they inheriently become more scarce over time.
1-5 years: motherboards slowly lose value as new parts come out
5-7 years: Motherboard prices go up as available boards becomes more scarce. Note some motherboard (enthusiast ones) can skyrocket in price like ones with overlockavle chipsets or Xeon CPU support. But In general, older motherboards suddenly go up.
7-20 years: motherboard prices again start slowly sinking again. Enthusiast mobos have a tendency to retain More value through this time. I still find high end LGA 775 mobs worth $50-80.
20+ years: same for the RAM. The high-end motherboards end up skyrocketing in value, and the average boards start to appreciate a little bit. Board prices stay high for the foreseeable future.
So in general, PC parts go down the first five years, go up as older but still usable machines need upgrading, then sink again after 7 years as older machines are replaced. Then after 20+ years they become “vintage” and start to rise again.