What is a good price for this Giant Defy 3 Carbon

This is a good bike but carbon is a bit of a delicate material for a first bike. You need to know exactly what you are doing when tightening every single bolt. Carbon can crack easily from overtightening. You should also check the steerer tube while buying to check for any wear. And inspect the frame very well for cracks or excessive wear.
Btw this bike is setup for someone very tall and unless you are very tall this bike will be way too big.

This bike was probably too small for the current owner, hence the positive angle stem and big stack of spacers. If it fits you, you’ll want to budget a bit of $ to swap out that stem, at least.

Where I live this is a $400 bike at most. It’s pretty outdated; still a perfectly fine bike, but not worth a lot of money.