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.
Suppliers can control (increase or decrease) their account's stock buffer from the supplier settings tab - directions here.
NOTE: Lowering the buffer 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.