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.
Obligatory Modern Standby mention, which will become important if you wish to move over to Intel's Alder Lake (12th gen) CPUs, as well as future AMD CPUs, possibly as soon as Zen 4 (Ryzen 7000).