Description 

Now the foreman in mines across Skyrim will sell ore that comes from their mines. They are business men, they would never turn up the chance at a Septim or two that wanders by...

Requirements: - 
Unofficial Skyrim Legendary Edition Patch

What exactly do the NPC's sell?

- A random amount of the ore available from their mine. Its available in large quantities, you should find anywhere from 50 to 150 chunks of ore. Iron mines also have a low chance to sell a small amount of charcoal.

- A random amount of ingots made from the local ore - significantly less than the amount of ore. Iron mines also have a random chance to sell a small number of steel ingots. (Don't count on it being available, but you might see the odd bar here and there)

- A very low chance of gems associated with their ore.
Gem/ore associations are the same as Complete Crafting Overhaul and are as follows:
   Iron = Amethysts, Sapphires
   Corundum = Garnets, Rubies
   Silver = Rubies, Sapphires, Emeralds
   Orchialum = Garnets, Amethysts, Rubies, Sapphires
   Gold = Garnets, Amethysts, Emeralds
   Quicksilver = Rubies, Emeralds
   Moonstone = Garnets, Emeralds
   Malachite = Amethysts, Diamonds
   Ebony = Amethysts, Diamonds

- An extremely low chance of selling a random assortment of spare tools such as pickaxes, torches, lanterns, axes, firewood, charcoal, leather strips and linen.

How much Gold do the vendors have?

The Vendors use the 'street vendor' levelled list and so will have 50 gold if you use vanilla merchant gold, and whatever your mods determine if you have altered merchant gold with a mod.
I know this is a very low amount of gold and not very useful if you want to offload your Fiery Sword of Enchanted Uselessness on the nearest NPC, but these are not wealthy people and they are more interested in making money off you than the other way around.

Will NPC's sell ore even if their mine is infested with Draugr/Mercenaries/Spiders?
No.
Or at least, they shouldn't. If they do please leave a comment and let me know which NPC is misbehaving and which quest you think should have to be finished before you can buy from them.

So who sells what?

Grogmar - Shors Stone - Iron
Requirements: Kill the spiders dead.

Gestur Rock-Breaker - Stonehills - Iron
Requirements: Take a message to Thane Bryling

Skaggi Scar-Face - Markarth -  Iron
Requirements: None

Beitild - Dawnstar - Iron
Requirement: None

Liegelf - Dawnstar - Quicksilver
Requirements: None

Kjeld - Kynesgrove - Malachite
Requirements: None

Ainethach - Karthwasten - Silver
Requirement: Evict the silverblood mercenaries

Pavo - Kolskeggr Mine - Gold
Requirements: Reclaim the mine from the Forsworn

Perth - Soljund's Sinkhole - Moonstone
Requirements: Kill the Draugr

Verner - Darkwater Crossing - Corundum
Requirements: None

In addition the following NPC's were already merchants, but you should now find them to have more ore available:

Filnjar, Blacksmith - Shor's Stone
- Increased iron

Gharol, Blacksmith - Dushnikh Yal
- Increased Orichalcum

Shuftharz, Blacksmith - Mor Khazgur
- Increased Orichalcum

Dushnamub, Blacksmith - Gloombound Mine

- Increased Ebony

What about Thorgar from Whistling Mine?

Thorgar is deluded about his prospects, the mine is almost dry. He's going bust, so the poor bloke doesn't have any ore to sell you.

What about Thane Bryling? She technically owns...

Nope. She's a wealthy woman. She's not gonna carry around a bunch of ore in her back pocket

This was inspired by - Mine Foremen Sell Ore by Kidhedera