Is schamberg's disease common or rare?

Quite common: This is a curious loss of the collagen that strengthens the small veins, and allows little ruptures when they have relatively high pressure, as in the feet from standing. I suspect it's usually not diagnosed. It's famously non-treatable -- the talk about "taking vitamin C to promote collagen production" is subscientific; you can try ace bandages around the legs and feet.

Schamberg's disease: Schamberg's disease is considered to be a rare dermatosis.