I would guess eBay etc has a lot to do with it but also as the cars are getting older,getting decent quality spares must be getting even harder to find without paying a high premium,therefore less mark up.
Unless it's a barn find I'd guess there's only really enthusiasts owning these cars now so they would know the value better.
I've got a garage full of Rs bits but they won't be sold on here as I believe a certain trader took umbridge to someone resurrecting a thread from 3 years previous and accused me of being a trader.
Despite numerous posts proving otherwise it seems certain people on here have more influence than we think.