512GB M3 Ultra is out of stock, not coming back, and there’s nothing like it on the consumer market. That’s the reason they go for so much.
The issue with the M3 chip is the compute performance, as it doesn't fit well the transformer architecture. This changed with the M5 (apple baked their own matmul into the chip), which would significantly speed up PP (and video/image generation btw), making the M5 Ultra significantly faster than the M3 Ultra and in practice much more usable. You can try to load Kimi or GLM on M3 Ultra, but it's not usable. Now the M5 Ultra is not out yet, but undoubtedly it will be a superior offering, and shilling 15k on 512GB version is actually reasonable (if it's every priced remotely around that tag).
My money is on small companies or affluent programmers experimenting with some new hobby / business model.