He was still in his teens when he began conducting research in the national archives on the subject of the library of the Aragonese kings of Naples, publishing his first two academic articles at eighteen.
In 1900, when he was engaged in the bookshop of Riccardo Marghieri in the fashionable Galleria Umberto I, he was not only already an accomplished bibliophile but had also gained the friendship of reputed intellectuals, including the philosopher Benedetto Croce. It was during a holiday in Florence that De Marinis met Leo Olschki, who realised the youth's potential and offered him a job. The two bookdealers parted ways in 1904, apparently because of 'financial incompatibilities'. De Marinis then founded his own bookshop in the old centre of Florence.
Success came rapidly. According to Neapolitan historian Romeo di Maio, he possessed the qualities that make a great bookdealer: 'sharp memory and intellect, daring audacity, very refined taste, warm communication, and above all, he knew how to listen'. De Marinis was indeed astute. In 1912 he obtained from Wilfrid Voynich a precious illuminated manuscript executed for Matthias Corvinus, King of Hungary; he resold this to J. P. Morgan for a colossal sum, to the regret of Voynich, who was unaware of its value.
In 1924, at 46, De Marinis decided to retire as a bookdealer, selling his business to Ulrico Hoepli for more than four million lire. He then acquired the sumptuous Villa Montalto in Fiesole, becoming renowned for his hospitality amongst intellectual café society. His guests included royal princes (Gustave of Sweden, Eulalia of Spain and Maria José of Savoy), and of course scholars like Bernard Berenson. More importantly, he was free to pursue his scholarly activities. He published his last article in 1964, at the age of 86.
Source: Romeo di Maio, 'Tammaro De Marinis', Studi di bibliografia e di storia in onore di Tammaro De Marinis, 4 vols. (Città del Vaticano: Biblioteca Apostolica Vaticana, 1964), I, pp. ix-xxxviii.