The thing I noticed and is not talked about is used books on Amazon. There was a golden time when they were effectively a buck each due to companies processing so many systematically and now you can't find a used book there that isn't a few dollars off new at best.

Does anyone know what happened here?

The market realized that a used book in good condition is practically the same as a new one. The rest is just supply / demand.

I buy my used (and sometimes new) books off of eBay.

eBay is a better bookstore than Amazon now.