What version of macOS do MacBooks come with?
It's only possible to check the default operating system and the maximum operating system of a refurbished MacBook when comparing them on RefurbMe. You can find them on the product page if you scroll down to Product configuration:
Most merchants do not share the actual macOS installed in the MacBook on their product page. However, you can ask the merchant directly after we have redirected you to their product page or assume that the Mac will come with the latest compatible operating system since this is the most frequent scenario.