Just a quick report:
I have opened up a couple save games, and noticed that I can’t sell any of my wet stones, dry stones, Greater wet stones and Greater dry stones.
I checked to see if they are in storage, and yes, they are in storage chests.
I thought I use to be able to sell them, but can’t remember now.
Edit: I am using the Better Storage mod, not sure if that affects this.
Those are marked as not sellable, not sure if intended.
Ok, but they have a 10g and 15g value in the Geomancer crafting menu. Its something to make for experience, but if I make alot they take up storage space, so I wanted to sell them. I know I could get rid of them by placing and destroying, but these things can cause alot of lag if not placed carefully.
I suppose they are intentionally not sellable.
“in theory” your Town holds a really important secret for Clan Amberstone (which most people don’t even know exists) which is the power of geomancy, so technically you shouldn’t go around owning/selling magical artifacts that people are not familiar with 
Besides, placing geomancy stones requires the magic of the geomancer itself so selling them are kinda useless if the buyer isn’t (or has access to) a geomancer, which we assume very few people do
Or at least that is how I understand it
As for having a value, all items do since they still have to count towards your net worth even if not having a value for market.
Most summon stones can be sold, just not the dry stone and wet stones. Also I have seen other Hearthlings able to place and pick up dry stones and wet stones, but only a Geomancer can place the other summon stones. It can be a little weird sometimes too, like when they put the dry riverbed landmark summon stone on top of a mountain in the desert biome, it spreads far out and has some completely floating land sections.
Awwww bummer
There goes my theory 
not to mention, that you can sell the heartbud seats and essences for some good cash money