How to make a modular buying system?

Having written one myself, start from a general "Product" class, and work from there.

For instance:

 public abstract class Product : MonoBehaviour 
 {
     public string Name = "Product";
     public string Description = "No Description";
     public Texture PreviewImage;
     /// <summary>
     /// Attempts to buy the product. Returns true if it was successful, returns false with a error message if not.
     /// </summary>
     /// <param name="callback">Callback.</param>
     public abstract bool TryBuy(out string error);
 
     /// <summary> Returns the price string of a product. e.g. "100 Coins" </summary>
     /// <returns>The cost string.</returns>
     public abstract string GetCostString();
 
     public virtual bool IsAlreadyPurchased()
     {
         return false;
     }
 
     public virtual bool Purchaseable()
     {
         return true;
     }
 }

Then extend it for each product you want, e.g.

 public class ShinyThing : Product
 {
     public int Cost = 10;
 
     //Class that loads/saves player coins to a file.
     PlayerFundsHelper fundsHelper;
 
     void Awake()
     {
         fundsHelper = new PlayerFundsHelper();
     }
 
     public override bool TryBuy(out string error)
     {
         Funds playerFunds = fundsHelper.GetFunds();
         if (playerFunds.Coins < Cost)
         {
             int missingCoins = Cost - playerFunds.Coins;
             error =  "Not enough funds. You're missing " + missingCoins + " :("));
             return false;
         }
 
         playerFunds.Coins -= Cost;
 
         //Save the results
         fundsHelper.SetNewFunds(playerFunds);

             //!!!Insert code here to add shinyThing to your player's inventory!!!
             
         return true;
     }
 
     public override string GetCostString()
     {
         return Cost + " Coins";
     }
 
 }

Finally, write a class that finds all the product classes you created in the scene and add it to a list. This one works by looking for Product Monobehaviours on children gameobjects.

 public class Store : MonoBehaviour 
 {
     List<Product> products;
     public List<Product> Products { get { return products; } }
 
     // Use this for initialization
     void Start () 
     {
         initialize();
     }
 
     void initialize()
     {
         //Now search for products, and add them to the products list.
         products = new List<Product>();
         products.AddRange(this.GetComponentsInChildren<Product>());
         products.AddRange(this.GetComponents<Product>());
     }
 
 
 }

Then to buy something...

 Store store = FindObjectOfType<Store>();
 //Attempt to buy the first product in the store
 string error;
 if (! store.Products[0].TryBuy(out error))
 {
       Debug.LogError("Purchase failed: " + error);
 }
 else
 {
             Debug.LogError("Yay the product was bought");
 }

Of course, all of this will change depending on the nature of your game.

To create your list of UI Buttons. Have another class that iterates over the products and generates a UI prefab in a scrollview. Then pass each product to a class on that prefab, and make it so pressing the button purchases that product.