Can be sold to NPCs for coins.
Can be sold on the Market.
Requires lvl 40 mining and pickaxe or multi-tool.
The coal in No Man's Land can be reached without combat, however there are a lot of Knights and Novice Knights to dodge along the way. Also note that any visit to No Man's Land is inherently risky because it's a PvP area. You might have to fight Bears and Grizzly Bears in Whiland to reach No Man's Land in the first place.
Can be sold for 250 coins.