I think the thing that bothers me most about the idea that this is a problem is essentially the issue of levelled loot, which was pretty much taken to its conclusion with Oblivion: as much as I loved Oblivion, it didn't take long before I needed to install an overhaul to make it worth playing, which did pretty much this: it gave you a chance of getting awesome items at a low level (as well as a lot of absolute tat at a high level).
The vendor in question isn't really all that easy to get to and there's already a built-in penalty in doing so in that all the magisters become hostile when you get to that point (or at least they did in my game when I tested it just now). It's always possible to exploit a game's advantages once you know they're there, but to attempt to then compensate for them being there is I think a huge mistake: there's too much of a risk of making the game either too bland or too awkward.
Then have him sell a Legendary for all weapon types that are randomly chosen when you first meet him. The lack of weapon diversity coupled with how strong Warfare and Rage are right now make having The Sunderer something not even worth considering. You just BUY IT.
Instead of having, say, a party without a Ranger, and he has a Legendary Bow in his stock might make one of the players pick up a few ranged points to use it. It could cause another player to change their build to take advantage of this really strong bow.
For example: First time I played the game, I had my mage dual wielding wands. I come across the vendor and he has his Legendary Staff. It's not a MUST HAVE because Staves aren't ranged outside of Magus Bolt, which can harm your own team if they're standing in water or are wet. It's not a brainless pickup. You need to consider what you have and if it will work.
Having a static set of legendaries just limits gameplay interaction.
