Why so expensive?

[Uprising]

I recently decided to downsize my life, so I decided to start selling off my black Library due to falling out of love with 40k lore (Only keeping The lord of Silence and Night lord trilogy).  While comparing prices, I discovered, I have several books that are going for $100 to $200, even a few $300 (looking at you Sythes of the empereor).  Why are these book so expensive?  Is it as simple as supply and demand or something out?  I am surpised to say the least as I don't need the money (Honestly was just going to throw the novels away at first).  

[Roomsky]

Black Library shortprints basically everything, and keeps very few books on their reprint schedule. It's purely scarcity, allowing people to list a lot of out of print items for ridiculous prices. It's one of the reasons scalpers flock to Black Library releases, it prints money if you can find the right buyers.

Honestly you could probably sell a lot of your stuff for half the secondary market price and still make a profit on it. Personally I donate any physical copies I don't want to the local used book store.