Varying prices of Star Wars hard cover novels

Inked

Was looking to sell the few Star Wars hard covers that I have but looking at online sale prices they seem to be varying quite a bit for some of them. Some are from very low to middling, some seem to have sold for astronomical prices .

Can anyone shed any light on it for me?

The books in question are;
Tarkin
Lords of the Sith
Dark Disciple
Aftermath
The Force Awakens