Serving websites is an area where capitalism’s promise of achieving efficiency of resource utilization through economic incentives probably actually works, via shared hardware.
This is a hobby and aesthetic thing, which is valid and interesting.
If anyone has some good data about carbon emissions of self-hosted vs shared hardware I’d love to see it.
Buying used also only works when not that many people are doing it. On an individual level, it is a good thing to do, but it isn't a solution. It does help foster an environment of maintaining and supporting hardware for longer, which is a better solution.
I could envision a scenario where solar self-hosters replace hardware at a rate so much lower than than hosting companies that it outweighs the additional hardware it requires, but I don't think it is the current reality.
The second-hand computer and second-hand panels have already been produced for someone else, their cost is sunk. But their value can continue with this project, or end with the hardware and panels going to a dumping ground.
If you buy a used RPi, it gives money to someone who buys new computers, and encourages them to buy more. It stimulates demand for the production of hardware, just like buying new stimulates the production of new hardware, even though the hardware you bought had already been built.
Yes, it is to a much lesser extent than buying new, but it certainly isn’t zero. And again, it only works on a small individual level.
It is a good thing to do, but it does not zero out the cost of producing (or running) the hardware.