Healer npc stays on the other side of a counter, o talk to them. "Hey, if you need some medical treatment i can fix you for 5 potatos" player clicks button agreeing and gives 5 potatoes
-NPC does /effect give @initiator minecraft:regeneration 10 5
Boom just bought medical treatment.

Pirate npc knows the location of a hidden treasure but won't give it for free, he wants 5 Golden ingots for it
gives 5 gold
Execute as player [X]
/wp create TreasureLocation minecraft:overworld x y z
(Using journey as reference)

I want to buy a flan terrain.
Talk to mayor npc, 20 diamonds for X size terrain in his city.
gives 20 diamonds
Npc executes: /flan giveclaimblocks @initiator X