Xandar is, while far from the most densely populated planet in the galaxy, still a strongly urban place, with its twelve billion inhabitants clustered on artificial islands. The urban design is spatious, designed to flaunt the wealth of an empire that terraformed an inhospitable planet into a testament to luxury as their new capital. The population is diverse in some ways - plenty of species are represented here - and horribly uniform in others - just about everyone in sight is well polished and respectable, and non-humanoids are incredibly rare.
Still, it's a really good place to pawn off rare and high value items like bizarre artifacts found in the depths of an ancient temple - the buyer for Ash's main prize is stationed here in a fancy pawn shop overlooking the main parkway, but she can probably unload a lot of minor crap on the assorted museums, only some of which care about provenance of artifacts and whether any laws were broken in obtaining them.
