Npcs have a relationship meter, being orange the worst, and blue the best. Some NPCs won’t sell certain items unless you have enough friendship with them. Once they reach a certain friendship you get 10% discount. If you have very good friendship (very close to the end of the blue bar but it’s not necessary that it’s full) then you get 20% discount (it’s the same discount for every character). They also will pay more money for items you sell to them with better friendship.

How to increase friendship:

-Complete their quests. (some gain more friendship than others, I show the final result in the pictures.
-If you gift them something they ask for.
-Smoking cigarettes (besides Ann, Helen, Steven, Tommy, priest Matthew)

Once you buy the items available in the store they will be out of stock out and they will be re stocked the next day.

Some gifts are more valuable than others and will increase more friendship. Usually the ones on top are the most valuable ones. For example, Moonshine and chocolate box are very valuable and increases more friendship, whereas vodka and chocolate bar are less valuable and will raise the bar slowly.

If you drink alcohol you can get on a fight with any NPC. This will decrease your friendship to the lowest point and they won’t sell you any items for several days. You can steal an item from an NPC but you can get caught and it will have bad consequences.You can also race an NPC to get some rewards (money and items) but they will be cheating so you have to give your horse some injections. You will get penalised if you lose.