I'm like you I have no Idea what they are going for on the secondary market, but I know people who are looking for them after the fact and I guess they can bring what are to us CRAZY PRICES since we bought them when first released.
I tell everyone who will listen Verlag Militaria are consistently publishing the best books in the world on the subjects we enjoy and if you don't buy them when published you will be sorry.
I've been a fan for over 20 years the books are well made and hold up to regular use, but I have to admit to having two copies of the ones I enjoy a lot just in case.
Sadly, they are not well known here in the US due to having no distribution which is a shame. I've heard they sell books at the SOS but if that is your business model for North American distribution which is the world's largest collectors' market, you're totally missing the ball.