Claudio, my understanding is not to focus on the value, offer up something that you think is acceptable.
I look at what I have that isn't junk that I dont want or need.....Congrats on your haul, thanks to Wall-Crawler for the amazing offer.
+1 on it is not about the value, at least it should not be. Just offer what you can, I know you are a good guy and we all have different resources. Honest effort is all that should matter.
As an FYI to the group, I bought some Avengers books off of Claudio awhile ago for a buddy of mine and they were awesome and as described!