eBay and Craigslist finds

Bearings were developed in Europe and have always been officially metric. The maths behind engineering is far easier if you use SI units. The leftover imperial components are related to tubing sizes, and everyone refers to them in metric anyway (eg 27.2mm posts).

I don't trust someone who refers to axle sizes in imperial to understand anything about component design beyond aesthetics.

What size is a quill stem?

What size is a pedal spindle on a adult bike? and on a childs bike?

What unit is used to describe the pitch of threads?

What sizes are all the common bearings used in loose ball hubs, bottom brackets, headsets and such?