Was literally thinking this, but you'd never find anyone who isn't biased to market prices. This is why it would have to be multiple people and require discussion and votes. Unless they're posting market averages which are gathered.
This leaves you with needing either:
A: an Economic Counsel which is unbiased (almost impossible)
or
B: A method to gather averages and for them to be updated/posted which can be done by someone
Secret option C: Both A/B with discussion in high value shiny price involving careful consideration of supply and demand, as well as other factors
Basically you'd need trusted members of the community to do this, preferably staff or others who understands the basics of supply and demand and can refrain from personal bias (most likely no one omegalul)
Even then, these are NOT solid prices that people would have to follow. These would be suggestive prices which people can be guided with.