I came up with this idea months ago(minus the overly complicated price thing - I get enough of the price going up and down from guild NPCs) but we never got any feedback on it. The problem with it is RMT. I hate to say that and I hate that SE will consider RMT before the player but eh whatever. I'd rather do it and then ban RMT after they abuse it than just give up in advance in defeat. Players really need to be able to find stuff when they need it, and if not the AH then the shops. Since crafters are already dumping stuff on NPCs, jacking up the prices at the appropriate shop for the appropriate items to motivate selling to them over a different shop would be the way to go. Players looking to buy then can visit those shops for the gear they want at affordable prices since it's just not worth it to the vast majority of players to lock up 7 of their AH slots with cheap slow selling items.

This is really the only way to go, and it's a matter of it being worth the time investment or not at this time. Probably isn't so it might be some time before it happens if ever.