I have been slowly replacing all of my paper books with Logos versions. In the beginning, I had a hard time justifying purchasing books in Logos that I already owned, and I had not figured out a suitable way to get rid of my paper versions. There were many times when I just could not afford to make a purchase through Logos unless I could get something back from my paper versions. I have sold a lot on Ebay in the past, but selling books did not seem feasible, so I tried selling them on Amazon. That experience has been wonderful. Amazon takes a cut, but you don't really have to do anything except add the book to the listing (which takes about 30 seconds), and ship the book off when it sells.
Example: I bought the two OT IVP black dictionaries and the Commentary on the NT use of the OT on special for Thanksgiving. I listed those three books in "like new" condition on Amazon and they sold within two days for about $27.00 each.
I hope those of you who are like me and need to get some kind of return for the books you are replacing will find this helpful. I would not be able to spend the money in Logos without selling these books to recoup some of my money, and the experience is quite painless.