when do shopkeepers change their wares? i wanna know so i can keep on checking the shops until i find what i want

I haven't gotten that far, but, the shopkeepers only seem to change inventories when you go to a new area; I don't think shopkeepers in the same area ever change their stocks.

eh, i found out that reloading actually changes their wares

Correct, unless I'm mistaken the shopkeepers have one section of stock that never changes, static stock, and one section of stock that changes when you reload or perhaps go into a new area.

correct

One minor correction: The stock changes whenever the map (i.e. the shop) is entered. Loading a savegame is equivalent to entering the map, so you can game that by saving in front of the shopkeeper, then reloading the savegame until the shopkeeper sells you what you want.