I looked into it a while ago but I gave up on the idea after realizing how few programs can actually run on one. There’s no “reverse VM” software that allows you to seamlessly combine multiple physical machines into one virtual one. Each application has to be specifically designed to take advantage of running on a cluster. If you’re writing your own code, or if you have a specific project in mind that you know supports cluster computing then by all means go for it, but if you’re imagining that you’d build one and use it for gaming or video editing or some other resource intensive desktop application, unfortunately it doesn’t work like that.

Edit: I dug up a link to the post I made about it in /c/linux. There’s some good discussion in there if you’d like to learn more https://lemmy.world/post/11528823

Check how nearby colleges and universities dispose of used assets. The state school near me maintains a very nice website where they auction off everything from lab equipment to office furniture. It’s also where all their PCs go when they hit ~5 years old and come up in the IT department’s refresh cycle. Only problem in my case is that they tend to auction stuff in bulk. You can get a solid machine for $50 to $100, but only if you’re willing to pay $500 to $1000 for a pallet of 10.