when I was evaluating my collection, it took one year to get it organized to the point where I knew value for most raw books. If they had been slabbed, it would have been far easier,
Hot books sell faster and slow books wait for the right person unless you want to dump the load. It's an enormous amount of work. I sold about 40 of the 950 books for nice money and the rest went for $3.75 each. raw but 6.0 or better. They were all silver age, mostly pre 1970. It included things like the AF15.
I wouldn't do it for 20%, knowing what I know now. I wouldn't do it for someone else at all. Your friend will still have cap gains to pay regardless of what you get. That's another 10%.