Can buttermilk be frozen successfully? Can I freeze unused buttermilk bought at the supermarket for later use? Can it later on be thawed and used safely?
Yes, you can freeze buttermilk. 

When thawed, the emulsion will break into solids and whey.   This makes no difference for baked goods; you can also use a blender to restore it to a smooth, thicker liquid if needed for applications like salad dressing.

See also:  


Video from America's Test Kitchen.
The Kitchn