I'd say about 75% of all CDs and DVDs I own are second hand.
Amazon (USA, UK and DE) is my main supplier, the vendors have almost everything. New CDs I buy often come from Japan, since they sound better. Also the Japanese are not afraid to release new SACDs, a format that seems to be banned in the rest of the world.
I insured them all as new CDs though. It adds up to quite an amount, and most home insurance companies only have a very limited coverage for audio equipment.