GBB, trading and banking sounds good. What are you thinking along the lines of an economy? Variable prices on materials that change from day to day? Like a roll 2d10 or something? As a guideline for human and NPC merchants, and maybe a PREDICT ECONOMY skill for merchants, that allows merchants to know prior to the roll? Maybe roll ahead of time?
((Also, for our GM, can the thieves steal from NPCs too or just from players? It seems strange that NPCs can sell us stuff and steal our stuff but can't be robbed... Also, since Vivilas has the stand ability, can he set up a "snake oil" sort of stand, and sell stuff so useless it doesn't even take inventory space to NPCs, things like splinters of wood "From the crucifix", a chunk of rock calling it the "Philosopher's stone" or whatever the equivalent would be for this world?))
Well to answer BOTH of your questions the NPCs will mostly sell 'crap' items. I don't mean they wont have a use, but they will be things like not only common consumables, but tools without abilities like a hammer or knife. OCCASIONALLY they will have ability equipment. Even basic, ability-less weapons and items will still raise your temporary level and help you when adventuring.
So theres two answers:
1. If and when this becomes an open simulation, every GM task will be pieced out to GODS, the king, guild masters and their talisman holders... so that if any one of these posts leave, one of their underlings can take up the role, for the gods that may be followers. Any story telling will come from player made quests, and perhaps appointed temporary GMs aka writers.
2. In the linear story, NPCs will offer more and more valuable items. The price wont change, but players will have more and more ways of making an income, tht progression will get easier over time... to combat tougher threats.
The Prosperity God would likely be the person that influence that.
P.S.: I may run two games, one open game, and one for linear sessions that either reset or continue after.
P.P.S.: When an equipment "teaches" its' ability, it will "lose" it and become its' basic variant.
P.P.S.S.: Please check out this wonderful free kingdom game!
http://www.noio.nl/2013/10/kingdom/