I've heard it claimed that the era of being able to do this (buy slightly old used server hardware cheap on ebay) is coming to an end because, in the quest for ever more efficiency, the latest server hardware is no longer compatible with off-the-shelf power supplies etc. (there was more but that's the part that I remember) and therefore won't have any value on the second hand market.

I hope it was wrong, but it seems at least plausible to me. I'm sure that probably fixes could be made for all these issues, but the reason the current paradigm works is that, other than the motherboard and CPU, everything else you need is standard, consumer grade equipment which is therefore cheap. If you need to start buying custom (new) power supplies etc. to go along, then the price may not make as much sense anymore.

I'm curious, what is the powe draw for such a system? Of course, it heavily depends on the disks, but does it idle under 200W?

I personally feel like I will downscale my homelab hardware to reduce its power draw. My HW is rather old (and leagues below yours), more recent HW tends to be more efficient, but I have no idea how well these high end server boards can lower their idle power consumption?

RAM! (And NAND SSDs too now, probably...)

When I was looking in October, I hadn't bought hardware for the better part of a decade, and I saw all these older posts on forums for DDR4 at $1/GB, but the lowest I could find was at least $2/GB used. These days? HAH!

If I had a decent sales channel I might be speculating on DDR4/DDR5 RAM and holding it because I expect prices to climb even higher in the coming months.

AMD also has some weird cpus like the 7c13 7r13, that are way way way below their normal price bands. You don't even have to buy used to get a ridiculous systems... Until 4 months ago (RIP ram prices). https://www.servethehome.com/amd-epyc-7c13-is-a-surprisingly...