Bicyclebluebook.com estimates the "private party range" for a bike in excellent condition:
2019 Cannondale CAADX Ultegra: $1,240-$1,277
2018 Scott Addict CX 20 Disc: $1,939-$1,995
So the Addict definitely is the better bargain at $1,800.
I've been riding and Addict R1 since 2011, and it's the nicest riding bike I've owned. I assume the CX frame is similar.