Update! [US] Amazon: Used RTX NVlink 4 slot $18 USD - minor imperfections
I was hesitant to post this, since it isn't really so relevant to eGPU's, but thought it was too good a of deal to pass up. I ordered one, so I'll let you know if it's a scam.
$18 is a still and might be too good to be true!
UPDATE: The box had a minor dent, but I couldn't find a single blemish. Looks like new to me. And it works!