Neighborhood change and reclaimed materials move together, in a way that's easy to miss. When an old district starts turning over, renovation follows, and renovation is where salvage both comes from and goes. The same wave that strips original fixtures out of one house sends someone hunting salvaged fixtures for the house next door.
Where the supply comes from
The supply comes from removal. A buyer renovating an old house tends to pull out exactly what a salvage shop wants: the tired original hardware, the painted-over mantel, the odd old fixtures that don't suit the new plan. Some goes to the dumpster. Some enters the salvage stream, which means a good part of what a shop sells was removed by people who didn't want it, during renovations that were sanding down the old house other people pay to rebuild.
Where the demand comes from
Demand comes from the same impulse a house or two down. The next buyer wants their place to feel old again and reaches for salvaged parts to do it: period lighting, real hardware, glass that matches the era. So a neighborhood can strip out its own fixtures and buy old fixtures back at the same time. The fixture that leaves one house almost never returns to it. It lands somewhere else, looking right for the period but cut loose from the room it came from.
Salvager's Note: If you own an old house, think twice before you strip its original fixtures. They're exactly what buyers two streets over are paying to find, and once they leave the house, they almost never come back to it.