I think that there is a nice ring of people that all bounce PC off each other.
Certainly asking Manticore is a given since being the "overseerer" of the Rares List Sticky, many people consider him an expert on pricing. I think that his involvement in the sticky is the LEAST of his qualifications. As I see it, his value lies in the fact that he's participated in SO many actual events and he's also owned so many different items that he's either bought or sold many of the items that we all might ask him about....
There are very many knowledgable people that are experts in their own right in certain areas. When I want to ask someone about rare clothing, I message Athelas.
Shoes = WootSauce
Server Births = Gunga
Rubble = Europa Trader
Plumbing = Restroom Cowboy
etc etc
There is also always an individual that is a prominent museum owner on most every shard now, so if you wanted to get some background info or the like, you could always go this route.
As far as valuing (10) versus (20):
We have all seen the value of items fall a bit recently, as I've posted about because of the flooding of the market with EM items in these ranges. Ask 10 people their opinion on this and you'll likely get a variety of answers.
To me, it is less about the number of items that drop and more about the name and graphic of the item. My priority of the standard system would rank importance this way:
(anyone that knows me, knows it's shard first, then...)
1. Name
2. Graphic
3. Hue
4. Usable/Unusable item?
5. Issue number
Either way, PCs are so very personal these days, it is quite a challenge to get consistent ones, but many people know after asking around for them for a while, who to ask, and why one can rely on them...