Bricklink is a US based marketplace exclusively dedicated to the sale of Lego. You can list individual pieces, full sets, Lego related items and also mini-figures (which is the official name for those little Lego people). It is available to shoppers and sellers from all over the world and clearly has a significant international following.

Founded in 2000 it claims to have facilitated in the region of 1 billion transactions and continues to grow. They say that as of 29/3/2017, that on the marketplace there are 12,911,612 lots for sale and a total of 472,289,523 from 41,624 sellers. It was founded by the late Dan Jezek and remains a private company. You can read a full account of the Bricklink story here.