It is a peculiar quirk of history that Benjamin Franklin ’s first diplomatic mission to England in 1757 concerned taxation. But he wasn’t in London to protest excessive Royal overreach, rather to demand taxes be levied on vast tracts of Pennsylvania land. The Seven Years’ War had started disastrously for the colonies and it needed funds to fend off French-allied raids against Pennsylvania’s western settlements. Franklin was contesting the Penns , recipients of enormous landholdings under a 17th century royal charter, who had long since retreated to England and were insisting their estates were exempt from taxation. Nearly a year into Franklin’s petition, he wrote a letter to Joseph Galloway , his political protégée who was a member of the Pennsylvania Assembly , outlining the state of his negotiations in England. That letter, signed and dated to June 10, 1758, is one of more than 150 items from Jay Snider ’s collection of Franklin memorabilia that is heading to Sotheby’s New York on June 24.…