I would of thought PC component sellers are shifting Cases, graphics cards, motherboards, cpu's, memory chips etc etc by the tens of thousands daily around the world.
Personal real life experience................ The only people I used to know who actually BOUGHT A PC from a shop were some women and old folks who knew nothing about computers at all and ended up with some Packard Bell junk.
Everyone else I knew bought "Parts" and either put them together, got friends to, or changed bits around.......
The numbers, well I don't know.
It's hard to work out when a PC is a new PC when you put a new motherboard in the old case, or a new CPU and GPU in your current machine.
I guess this concept it probably totally alien to most apple users who treat their computer like a microwave, fridge or toaster you just buy and use from the store.
As I say there must be tens of thousands / millions? of pc components being sold around the world daily to people ungrading/changing/building PC's and it just stupid to pretend not of this exists.