Gumroad balance design question

gumroad_design

data model:
  
  seller:
          has_many :products

          attributes: balance
  
  product:
          has_many :purchases
          has_many :buyers, :through => :purchase
          belongs_to :seller

          attributes: price, seller_id

          add index to seller_id

  purchase:
          belongs_to :product
          belongs_to :buyer
    
          attributes: payment_accepted(boolean), paid_seller(boolean), refund_requested(boolean), refund_processed(boolean),     purchase_date, product_id, buyer_id

          add index to product_id and buyer_id

  buyer:
          has_many :purchases

          attribute: balance


I added an index on seller_id on the product model because you will costantly be looking up who's selling the product. 
I added an index on buyer_id and product_id on the purchase model because those will be frequent queries.

There is a relationship between all 4 models. You can query all of a seller's products.  A product can be purchased many times.  Each purchase has the product information as well as the buyer's information.  

To pay a seller, take the sellers products and query all new purchases up to a week before the pay day if that's the pay schedule.  You can set a different schedule by specifying the pay schedule.  The purchases' product_id will need to match with one of the products that belong to the seller.  The amount will be totalled and added to the seller's balance.

To refund a purchase, the relationship on purchase model allows you to find both the buyer and seller.  Deduct and add the appropriate amount of money from the buyer and seller to complete the refund.