In the thirteenth century the Venetians were masters of the sea, and some of their wealthy merchants- as the Sanudos, the Justiniani, the Grimaldi, and others- erected principalities in divers places of the Archipelago, which their descendants enjoyed for many centuries. These self-created princes were called “royal merchants.” (Warburton.)
Glancing an eye of pity on his losses, That have of late so huddled on his back, Enough to press a royal merchant down.
Sir Thomas Gresham was called a “royal merchant.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894