This is the sort of thing I typically do a brief play-by-post thing to handle. It's extended down time and if you don't have an idea to turn it into drama or an adventure of some sort, it should probably be handled like extended down time.
But that's probably not helpful.
You could have the players learn about previous attempts at a similar idea and how they failed. Introduce characters that would take advantage of the trading post or attempt to abuse it. Recommend an abandoned prison/dungeon/mine to use for safe storage and introduce some history about the area and possibly throw in some plot hooks like a sealed up back door, evidence of an old crime, or hidden untapped wealth. There could be difficulty securing rights to trade, or oppressive taxation. The trade center could also reignite some political feud that would need to be resolved by picking a side or cutting a major faction out of using the center.
Ensure that the situation still contains conflict that has to be overcome, and that physical violence can't help.
I just finished a two-session run of Swords of Dragonslake (from Dungeon Magazine 141) and there was a total of one combat between the two nights. This, because the interpersonal relationships between the NPCs has to be unraveled to figure out the central mystery. (It's a missing-persons case.)
Basically, you have to write a circumstance that is filled with things the PCs need to do (skill checks), but make it so that violence will prevent them from success.
Combat needs to be prepared in advance more than social encounters. Sure, you need a plot and some names. Then I read a few chapters from books with good characterisation the day before and wing it.
When running the game make sure the situations call for more skill checks than just perception & diplomacy - knowledges or linguistics to drop hints, stealth, climb & acrobatics for sneaky stuff, etc. Most characters should be what they seem so that you and the players can keep them straight; a few can be otherwise.
When you've finished make sure you make some notes of what's happened so you can remember if you come back to the same characters later.
Social games can be difficult to run and play in. How smoothly it goes will all depend on how comfortable the players are. Some people don't like to talk much during gameplay, while others love to talk and can keep it up for hours. Die rolls will happen with this type of scenario for sure, but actual in-character talking is extremely important. Some people, like myself, aren't naturally eloquent and convincing while speaking through imaginary characters and don't like to talk in-character so much. Make sure that all of the players know what is type of game this will be and what will be expected of them before you start, so that you can allow the players who don't want to play a social game can drop out if they choose. Also, character backstories will be more important in these types of scenarios, so make sure that your players have good backstories to help flesh out their characters.
In combat encounters, the opposition generally dies and is thenceforth of no further concern. So you don't have to keep notes. OTOH, people in social encounters come back. They have motivations and personalities. And friends. And families, enemies, needs, grudges and weaknesses. And resources, appearances and dwellings. And, of course, statblocks. You will need to keep notes on all this. Like an encyclopaedia. And you'd better hope the players can remember it for themselves.
they were warned don’t build only for combat as it will handicap your character.
Kind of feel like this should be the warning on the front of the chapter about character creation. :)
As people have been saying, there has to be conflict, and there has to be risk, for there to be interest. In combat, it's obvious because enemies try to chop your head off. Out of combat, you need to create the same conflict and risk - a fire in the granary needs to be put out, or a romantic relationship needs to be repaired without either party aware that intervention happened.