Currently, the Carrack has an upgrade value for "old" buybacks of $500 and not $600. There are a couple conditions: You must already have a ??? to Carrack in your hangar and you must have purchased that item before they updated ship upgrade tool (I think they switched around 2019 Anniversary...
There are a couple ways. You have old $0 ccu's. For example, Redeemer to BMM to Carrack gives you a Carrack for $250 (50% off) Or you bought a lot of IAE warbond upgrades. They each had about a $15 discount.
We are all waiting for Turbulent to make them available. Everyone is in the same boat.
Any items bought during sale and melted can not be current bought back. But items bought before sale and melted can be bought back.