Fanaelialae
Legend
I don't know about this. Maybe if some of those books had a small print run.Buy them on the cheap and then when they go out of print, sell them at a premium. It will be money gained.
Even if there's a high demand in 20 years, you have to take supply into account. Most old D&D books sell at or below the price on their cover IME.
For example, the cheapest copy of The Red Hand of Doom is listed on Amazon at $56 (which is crazy, because you can buy a POD copy from DMs Guild for $20). Compare that to a different adventure from around the same time, The Sunless Citadel, which is under $16 on Amazon. An original 3.0 Player's Handbook can be had for under $4.
The discount helps, but as far as I'm aware, PF1 generally had a large print run, meaning you won't necessarily make much money, if any. And that assumes that in 20 years, POD won't have advanced to such a degree that it's extremely cheap to download the PDF and just print a new copy.