You will have to be extraordinarily lucky to find a carbon frame bike at a garage sale or thrift store. I've found many a good bike at such places but never even seen a carbon frame bike for sale. You are looking at the top end of the market so it is not surprising that what you find on CL is expensive. I think some sellers are nuts with what they are asking for what is often custom built to fit them but expect a secondary buyer to pay nearly what they paid for it originally.
My Ten-Speeds has a huge section devoted to this pursuit.