I have found a script of this somewhere, but yeah, im not here to ruin your work, and i praise you for finding a new way to do it, although i think it adds a lot more work for people who are using XAS or a lot of items like i did in my project, where i have to duplicate item (that is ok), but for weapons, i have to do a bit of scripting for it, by that i mean add it as a new weapon, and give it same properties and give it a new ID, not that it's too hard, but it just adds a bit of work.
I once again want to tell you that i'm not here to ruin your work, i just want to throw some extra information at it, and for people using the vanilla, this will be excelent, and if one say it once, one say it again, i praise your work
Then, to put in an idea for other people, as well as you. One could make it so that you have to do a side quest for a shop keeper, a friend of him or his family to be able to unlock the 50% off shop.
Then again, you can make turn it around, and instead of having quests, you can have reputation, which can either be Town, Guild or something simillar, just use your imaginations / game story, and make some space, and think of all the possibilities you can use this for!
Well, enough talking from me, keep up what your doing mate