If all else fails, you can /spawn_creature, select the npc that appears, and use the command:
/creature_script npc-trading-guild.lua
The resulting NPC will not have the Trading Guild skin, but when talked to he'll act exactly like a trading guild member... There are other /creature_*blahblahblah* commands that let you send him places (specifically, the inside of the shop, in your case) but you can look those up in the pinned command thread; I forget what they are at the moment.
But as Wolfe said, the NPC should just appear from the portal stone stack of computer blocks in the center of the shop.