We all know that people buy off their website store, why not implement it in-game, that way you don't have to exit or minimize the game to buy from their black market.
It would make a whole lot easier plus people can search things they need that can't be sold on the Auction House and buy the items
There is no need to modify the client, just add an NPC who would sell what is available in the store. However, I think it would be very time-consuming.