Something like that would be a pretty complex edit. Plus there are lots of questions and variables if you were to setup a system like you speak of. Should the price also fluxuate if someone were to say sell 2,000 of the items to the vendor? If not, the the economy system will quickly become screwed up for your entire shard. Some of the items that most shards sell on a vendor stone like this system, can usually be easily crafted. Meaning that players that were willing to sit and do the work, could quickly become rich by macro crafting a bunch of items and selling them back.
I think to keep the focus on the economy, once something enters the economy (either from resource gathering, crafting, buying, ect.) That it should then remain on the player market, meaning they should either trade with a player, sell it on a vendor, or put it in an auction. This allows prices to easily raise and lower as some items become the next big thing. With a set in place system, it would be harder to control the economy of your shard. Which usually makes or breaks a shard. No one likes a shard where you have to rely on guild hand outs, or events for anything decent. And no one likes a shard where you can quickly macro and become filthy rich. It's almost a fine art finding the perfect balance between the two.
Don't get me wrong, something like this could be done. I just think that if someone was going to take the time to do so, they should think it all the way through first. So they could also deal with problems like I mentioned, that would most likely show up later. So yes...if you're up for the challenge, you could do what you want. Just keep in mind that it won't be easy if you hope to make it cheat-proof and idiot-proof, also so it doesn't mess up your shards economy really bad.