Normally, for second hand stuff, I like to buy from friends. This way I know what the parts have gone through. The C24 wheels and 9000 group set (minus the crank set) were bought from a friend. Crank set, saddle and pedals were all bought on second hand FB groups.
Ruten is the second hand or business 2 consumer website. However, it's a pain in the ass to sign up/buy/sell. I would use it as a reference for price matching.
There's one major second hand FB group that people post some very legitimate components. Check this FB group "2017 你 愛 他 全新 二手 商品交易平台". This is where I bought my crank set and saddle. Your FB profile pic has to be cycling related to get in. A fair warning, admins are pretty strict on the rules, make sure you read them or have someone translate them for you before buying/selling.
The group is a buyers market. Sucks to be a seller, but it's a good way to get rid of higher end components if you don't want them sitting around. Shoot me a PM if you have any other questions. I'll be happy to chat about it.