per my reading of the article, the protection is only needed for a few seconds, while payment is being processed by the payment system.

so the row is inserted when Payment is initiated, and row is deleted when Payment succeeds

  What is oversell protection?
  Reserve: When payment starts, we mark items as reserved (a short hold, e.g. several minutes).
  Claim: When payment succeeds, we permanently deduct quantity from the inventory ledger (source of truth).

but that system could be easily improved to reserve item when user Adds item to a cart, to prevent scenario when user adds item to a cart, goes through checkout, and after initiating payment gets "soldout error":

  1. Let user add item to a cart by default (happy path)
  2. Initiate async check in the background for SKU and quantity
  2a. The check sums up rows for all SKUs and compares to Inventory table (very cheap check since its done to only active shopping carts)
  3. After few seconds the check comes back, and we let user know that item is soldout, before/the moment user goes to Checkout.