I'm suggesting that they offer one of each item slot type of each level and one of each weapon. Their inventory would replenish as usual. No shit that you don't have unlimited money, i'm not suggesting that you would go buy entirely new sets of gear every level or even buy a new weapon in this way every level. You buy something when you really need it, just like you do with every other item being sold by every other vendor currently in the game.
The difference with this is that you'll always have an item of that slot available to gamble on if you choose to. Spears are the best example of the hardest to find item and you'll almost never be using a spear of appropriate level without something like this. We've all had times when we had gloves or boots or whatever that were 5 levels behind and you become more inclined to buy mundane white equipment to replace them just for the armor.
Ultimately i'm proposing a gamble. I'm not proposing a way to get the best gear that will reliably be good all the time. Like it or not, you don't have to choose to do buy anything from the vendor.
