Apart from his work at the British Museum, Doubleday was a dealer and a copyist of coins, medals, and ancient seals. He sold sulphur and white metal casts, the former coloured in different hues, at his establishment, which, located near the British Museum, may have helped facilitate his employment there. He also sold curiosities, such as cabinets, snuff boxes, and lead seals purportedly made from materials taken from the charred ruins of the Palace of Westminster with the consent of the Commissioners of Woods and Forests, and pieces of wood said to be from a tree planted by Shakespeare. In 1835 Doubleday advertised for sale copies of 6000 Greek coins, 2050 bronze, 1000 silver, and 500 gold Roman coins, and 300 Roman medallions, in addition to other antiquities and what Doubleday termed "the most extensive Collection of Casts in Sulphur of ancient seals ever formed". By 1851 he had casts of more than 10,000 seals, and at his death it was said that he "possessed the largest collection of casts of seals in England, probably in the world." This comprehensiveness led to his contribution to the 1848 "Monumenta Historica Britannica" of a descriptive catalogue of Roman coins relating to Britain. More unique pieces he sometimes exhibited, either himself or by loan to Sir Henry Ellis, to the Society of Antiquaries of London. Doubleday's casts came from a range of places; on good terms with a variety of institutions and collectors, he was permitted to take casts at will from the collections of the British Museum and the "Bibliothèque nationale" in Paris.