Although, during Dr. Rosenbach’s career as a dealer, scholars and experts in bibliography had been buying books – men like Eames and Wagner and, of course, Belle Greene – they were the exception rather than the rule in the world of collecting. The giants had been the Morgans, Huntington and Folger, the Wideners, Owen Young, the Clarks, Clawson, and Jones, intellectually interested and to a greater or lesser degree knowing about books, but not technically or professionally expert. They depended on librarians and bibliographers of varying degrees of experience and ability, but chiefly they depended upon the dealers from whom they bought. Obadiah Rich and Henry Stevens had set a pattern in the nineteenth century. The dealers were supposed to know all there was to know about the books they were selling; they were supposed to be the experts or to have consulted the experts. Dr. Rosenbach had achieved his success because he did, in fact, know more about most of the books he sold than his customers and more than most other dealers.
    --- Rosenbach: A Biography (by Edwin Wolf 2nd with John Fleming, Cleveland and New York, The World Publishing Company, [1960], p.398. [Hereafter, cited as Rosenbach.]

Beyond Dr. Rosenbach’s extensive, almost infinite, expertise, there was the factor of his considerable charm as a salesman, which he put to good use throughout his career and which has often been remarked upon.

…Would Mr. Harkness [a potential customer] do him the honor of having lunch with him? The bait of a lunch sounded harmless. There is no word which describes that which stimulates book-buying as “aphrodisiac” is used for that which stimulates venery, but if there were it would have to be used to tell how Dr. Rosenbach seduced his clients. It was, of course, the man’s own charm and his apt mixture of anecdotes and scholarship. It was his assurance in the quality of his wares. It was, in addition, the latent acquisitive instinct of the buyer and the inescapable fact that money was only money, but books were --- well, books!
    ---Rosenbach, p.320.