The more you trade books with a librarian, the better deals you get

Each book sold to a librarian should increase a chance of getting rare (mending) or high level enchanted books when he reaches the next level. Thus, players wouldn't have to rely solely on rng and place/remove lecterns just to get a favorable enchanted books trade. Giving players more control over the deals makes trading much easier, but also requires more actions (making books to sell) than repeating the same dull procedure with profession blocks.