I see the option to import a master page, but no option to export one. How does one export master pages from a project?
To my knowledge there is no method offered to export a Master Page. At least, not from the UI.
When you create a Master Page, you are creating a file bearing the extension of .HTT. When you are viewing the Master Pages pod, try right-clicking one of the .HTT files you see. You should see an option on the context menu labeled "Explore". Click that option and Windows Explorer should open at the file location where your Master Page files are. Note that you will see other project files there as well. Master Pages are co-mingled with other files in the project.
Locate the .HTT file of choice and copy it where you wish.