First time you sold some of your D&D books.
I have never sold a D&D book. In fact, I have probably bought some of yours. I did give my friend a copy of my 4th edition Beginners Box. She had given me some board games from her collection and had expresses some interest in trying out D&D. I had no plans for playing 4th ed so I sent it to her. I considered it an act of proselytizing.
