tl;dr (I don't want you crayon-eaters influencing my first-blush opinions.)
EC, I commend you, but that's a pile of liability that I'd never even consider to take on. Blowing up a tube of primers. Some idiot making bad ammo and then "blaming" you or the shop?? Moron loading ammo into his gun while in the shop. I can think of a half-dozen "yeah I don't want that" type of situations.
Sure, the % of window-lickers on NES is surprisingly low. (lol) But you're going to get window-lickers. So you'll need to be there - or someone will - to watch over at all times.
As my #2 daughter (who turned 20 yesterday) used to say when 3 yrs old, "NOFANX!"