Magic Items shop - how to handle them

I´ve been playing RPG for a long time and one thing that always bothered me are magic items shop.

Lots of questions come to mind:

- who creates them?

- how the seller got a hold of these items? He must be epic level to sell a vorpal sword

- who protects the store?

- why the BBGE never tried to rob such a store?

- how come we have 1 (or more) per town?

how do you handle this to prevent the players and the world to be flooded with magic items?

if there are no shops, how the PCs acquire magic items?