I think what you are doing is perfectly fine, of course. As long as you make the effort to permanently forget the contents of the books you give away.

Given my memory, that's not at all a problem! I wonder if I should pay Jack each time I re-read one of his books. It would be worth it!

If we feel guilty passing on books, or buying them at auction for $50, we could, of course, pay Jack a percentage of those transactions also...