I wonder where the prior-generation servers end up once well-funded companies upgrade

E-waste recyclers because they can handle bulk.

Suppose 500 servers get replaced. To a first approximation, nobody who needs 500 servers is interested in 500 old servers and if they were they already bought 500 servers or are waiting for a better deal or aren’t ready to move out their 500 servers to make room. Recyclers have the real estate to receive 500 servers and experience disposing 500 servers.

And there are not 500 hobbyists lined up…and who wants to deal with misinformed consumers? There are all sorts of incompatibility nuances with complex computer hardware.

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Advice: If your hobby is gonna be training models, just build a box. If you want a server rack hobby, then buy a server rack and be ready to pay the dumb tax.