Why is it important if the vendors stay? The reputation requirement pertains to equipping the items, not buying them - just like any other requirements on items.
If you do not have Champion rep, you physically cannot purchase them. Much like you can't purchase the Blue/Red Enclave armor without being Friend, same thing as not being able to buy War Hero/Elite War Hero pvp gear until you are actually level 50. Even if you have the currency to buy the items, if you do not meet all of the prerequisites, you cannot even purchase them and hold in your inventory.