There are many vendors in Diablo III, but only one of them is a child – Squirt. (Well, before the expansion, anyway). Where is this kid getting the goods she is selling from?
The Barbarian vendor in the Keep gets his goods from caravans that occasionally make it through the battlefields. Some of the other vendors travel into the sewers or aqueducts to pick up “trash”. Players can easily find this out because those NPCs (non-player characters) clearly state it. Squirt, on the other hand, never specifically mentions where she gets her goods from.
Sanctuary is a place of scarcity. One might expect that in a world that has been overrun with both demons (and some angels) who are constantly trying to kill off the people who live there. What started out as a paradise, and an escape for renegade angels and demons who wanted to opt-out of the Eternal Conflict, has become a dangerous, desperate, place.