Russia had established a presence in Alaska during the first half of the 18th century and had skirmishes with the indigenous Tlingit people already there. So I guess that gave them the right to sell it? Czar Alexander II sought to unload the territory for some quick cash and found a buyer in US Secretary of State William Seward. Czar Alex also considered Alaska to be difficult to defend in case the United Kingdom attacked.
A price was agreed upon on March 30, 1867 at a price of $7.2M. The treaty became effective October 18, 1867. Alaska became a state in 1959.
