Why is buying ranarr so slow on the G.E. now

Depending the price you will buy them will take a lot, since lots of people buy it.

Probably because people are not selling...

Sure, people are selling, but lots of people also buy at max for prayer potions. That's why they're so profitable, and why they cost so much more than most other herbs.

I nearly always buy my ranarrs at max. Takes too long buying them at GE price.

Could also be that most people who get them are high enough herblore to make potions with them. And prefer that to selling them.