To avoid overselling, there is a default stock buffer of 5 units for all partnerships.
Carro monitors the supplier's inventory levels every hour and marks products as "out of stock" on the retailer's site when the inventory falls to 5 units.
You are able to change (increase or decrease) the stock buffer. However, it does increase the risk of overselling - Shopify limits Carro to sync inventory between partners to once per hour. If it's a popular item, it could oversell within that 60-minute time frame.