Buying stores is a system where as a player automatically buys items as long as they have loot and zenies to buy them.
There are two ways of obtainingor . One in for non merchants and other on for merchant classes.
For Non Merchants
Talk to Black Marketeer inside the navigation mentioned above. He will offer to sell them for 500z per piece and at the max of 10 pcs at a time.
Talk to Purchasing Team inside the navigation mentioned above. He will offer to sell them for 10,000z per piece. After that, he will teach you to open a buying store and give 5for free. Later, you can buy more from him at 200z at max of 50 pcs at a time.
Use them inside market map after getting a vending spot from NPC. The player must have at least one item and enough zenies to bulk buy them. Only misc and non brewed consumables can be auto bought from this system.
If you miss some items such they want to add to the store but it's not possible they can write a suggestion to implement.