But I don’t understand exactly how you want the merchant thrall thing works.
Usually, playing PvE-C, I trade with other players.
If the merchant thrall can pay me in bricks or other resources it will be good, but also… a shortcut for a survival game.
If it pays me in gold or silver coins… well… nice idea for roleplayers but too few merchants in the game to be useful for other means than making alchemical base (99% of the money I collect are used for this mean, and not for trade with other players or buy things from merchants).
There are “auctions” in other MMORPGs. You come to Auctioneer, put the amount you want to sell, set your price in some game currency. Others look through the lots and buy what they want, after that you get your money and lots that were not sold. Pippi mod presents own game currency and Social Merchants that work in the same way. It can be interesting for dense-populated servers I think.