@Victor: I'm aware of the changed stock at level time. I also noticed via saving and reloading before talking to vendors before they restock (known to me as 'vendor rolling') is that their magic items (Uncommon, Rare, etc.) are always of the same slots (rings, hats, etc.) with randomly-generated modifiers/affixes for their type. This means that if a vendor has 1 magic ring in stock, vendor rolling won't change that, but vendor rolling can change what that ring does. The first time, that ring may be +1 Summoning. The second, that ring may be +1 Necromancy. And so on.
Yeah, that is basically the same, if you get an item via Lucky Charm.
So you could reload if they have a unique belt with stats you don't want, but ofc doesn't work on non uniques.