I was already thinking of doing this, but instead of a shop, I propose to do it through keywords (like: 'Buy 1 key [gold amount], 'Buy 5 keys [gold amount]' etc). This way, master trader bonus is taken out of the equation. Deal?
That would be a good change. This should be a flat rate for all players, regardless of profession or bartering mod. It's just a good way to remove some gold from the game. Without a solid gold removal system, the games economy can get really shaky.
Can't really be double the price though, buying stones at Jeremy's is cheaper so you could work around that. Estone with MT is 93g for example
I think double for the convenience. If saving 27g a stone is worth the headache of hauling and handing in stones, players will go that route and save some gold if they have access to a mt. Those who feel it isn't worth the 1000g or so per inventory will just straight buy the keys. It's about options and convenience and for that convenience I think this way needs to be the most expensive route for a player with a MT.