Not sure of the functionality of this - but it would be interesting to have vendors that only deal in certain price ranges and cost accordingly.
Not sure how much vendors should cost in that regards, but breaking it down so that low cost items (or a small qauntity of items) can be sold via vendor that is affordable by a level 20 main. However, a higher priced item could not be sold on this vendor (preventing higher levels from getting a vendor cheaper to sell better gear).
I wouldnt begin to guess what a fair break down would be but just an idea to toy with.