How to only export products that are available for selling?

Under Feed management → Product exclusion rules, two options control out-of-stock items:

Export out-of-stock products: when disabled, affected items are excluded.
Export only out-of-stock products available for order: this additional filter applies when out-of-stock export is enabled.

Also check each product's out-of-stock behavior and PrestaShop's global setting.

This selection differs from backorder availability: see exported values.