How do I value some very rare police buttons?

Hi, I’m after some advice on a set of 3 buttons I discovered whilst metal detecting. These are Aboriginal Native (mounted) police buttons, dated around the 1850s, I can’t say much about the area, I believe they may have been burned as a whole uniform and only these remain with a buckle quite destroyed.
I collect military buttons and badges that I find, but the history is super cool on these, I’ve sold them to a friend in the past and regretted it then purchased them back for the same amount thankfully. I’ve been offered a decent sum of money by a few people lately (a year since I found them) and I’m stuck on what to do, part of me thinks they’re worth a bit less than the offering but I wouldn’t know, they seem extremely rare and hard to get, especially in this condition.

Would anyone have any advice on how to value them? Or what to sort of expect for a serious buyer? All the museums I’ve contacted are either clueless or want them donated, as they are aboriginal related it’s a kind of controversial thing anyway.
Thanks in advance, photos below.

 

I take it you're in Australia? Have you tried contacting good auction houses that specialize in this type of thing?