One of the most famous and popular spots in Sigil, the Grand Bazaar is an enormous network of open air plazas connected by covered walkways. Being the crossroads of the entire multiverse, Sigil attracts merchants from every conceivable place, and all of them converge on the Grand Bazaar to sell their goods. Merchants of every stripe rub shoulders in the Bazaar (sometimes literally, given how crowded it gets). Dwarven weaponsmiths are crammed up next to Elven fletchers, around the corner from a fiendish scroll merchant who shares a plot with a stall selling saintly artifacts and holy water from Mount Celestia. Creeds, faiths, wars, and racial hatreds are all temporarily forgiven, all races united by their common pursuit of gold.
The Grand Bazaar is the informal headquarters of the Free League; a contradiction of sorts, as the Free League lacks any clear organization or leader. Nonetheless, in an impressive display of spontaneous order, a loose network of charismatic and influential Indeps work together to keep the market running smoothly. Their network of contacts gathers information and passes messages along with legendary speed, ensuring conflicts over stall spaces or stolen goods are settled as quickly and as smoothly as possible. Most of the merchants have no interest in rocking the boat, as the more time they spend arguing is less time they spend making money, so everything always seems to work out in the end. Not that there isn't the occasional bureaucrat or ambitious landowner interested in muscling in on the Grand Bazaar, but persistent troublemakers always seem to meet mysterious and deeply unfortunate ends.
The chaotic and multicultural nature of the Grand Bazaar also makes it a frequent nexus for portals, and those in the know can get to almost any Plane in the multiverse from inside the Bazaar. That works in reverse too, however, and Primes and Planars alike frequently end up in the Bazaar whether they intended it or not. The Indeps and the merchants don't seem to mind, though. Whether its minted in Arborea, Gehenna, or the Prime Material, gold is gold, and that's all anybody really cares about.