Previously $139, Amazon has now dropped the price to $119, matching the now-expired eBay deal.
This is an excellent-quality power supply with Japanese caps and hydraulic bearing fan.
As for the differences between the RM750 and the RM750x, TomsHardware says this:
it uses Chinese Elite caps and a Hong Hua rifle bearing fan. The slightly more expensive RM750x comes with Japanese caps and the high-quality NR135L fan. Although those differences are notable, still the provided warranty is the same, at ten years, for both models.
Corsair released a new 2021 model so it looks like these are being cleared out, so stock probably won't last too long.