Quite a few hobbies start with the nuts who collect what most people view as useless garbage. Then people hear of others doing the same thing and there is less stigma to it. Then you get more people in the hobby and prices start to rise above free. Eventually people start posting on Facebook and Youtube about it and it starts being a craze where people with money buy everything "cool".
P4's will be worth something down the road because of the vast amount of people who started using computers on that hardware and want to do it again 20 years from now after 99% of it has been recycled. I only have a couple P4 machines because the chip never interested me, but I have a ton of Athlon XP/A64/Opteron hardware of the time period that did interest me. I collected 3DFX before it was cool because I was an early 3DFX adopter and later on I just snagged the models I never had when they were surplus junk.
There are people now who love collecting Packard Bell machines when they were considered junk and a joke when new.
I do think Ebay prices for rarities can be manipulated but not the more common stuff that sell all over. Billions of dollars of old hardware gets sold on ebay every year, so those prices are more accurate then what one item sold for twice on Amibay.
Collector of old computers, hardware, and software