The ancestor of the shopping mall the marketplace was the business hub of most urban areas until very recently.
At its simplest a marketplace is just a hole in the cityscape - an open area where traders can assemble to sell their wares - and in many places, particularly those too small, poor or primitive to have a permanent merchant population, it will remain just that.
Most places upgrade it pretty quickly though - it normally gets paved, to stop it turning into a dust bowl or a mud pool (depending on the season) and drained for much the same reasons. Many places will also construct (semi)permanent structures to shelter traders and/or shoppers and in hot countries (or ones with a lot of rain) the marketplace may be entirely roofed over for the comfort of all concerned. A full-on covered market can turn into quite an impressive structure - often a labyrinth of corridors that stretches over a significant distance which can be confusing to navigate, especially when busy.
Eventually a city may become big enough to get an assortment of market places for different things - dry goods generally like to be somewhere away from the livestock market for example. In a port city there may be one market near the docks specialising in imported goods and another on the landward side of the city for produce from the hinterland. Where slaves are traded, the slave market may or may not be a separate venue.
Markets can be daily, weekly, annual or at any appropriate interval - obviously fresh goods benefit from daily sale, luxuries might only sell once a year when the merchants return from distant parts with their cargo. An annual market (especially in the middle ages) can be a huge deal.
Other amenities often follow - a water source is common, whether a well or a fountain - and some kind of sanitary arrangements are usually welcome as well. The buildings around the outside the marketplace will often end up playing host to inns, taverns and food shops attracted by the passing trade - this may be as well as or instead of stalls selling the same thing.
As well as trade and commerce a market place traditionally serves as a hiring place for casual labour and quite often a camp site for any travellers who can't find accommodation elsewhere (cited repeatedly in The Bible and doubtless elsewhere).
Tax officials, law enforcement (like a Court of Pie Powder) and money changers will also accumulate eventually and any gathering place will attract entertainers, rumour mongers and, in all likelihood, public trials and punishments as well. In any state where the opinions of the public matter, the marketplace will usually become a centre for public speaking and debate. This can get so serious that somewhere like Rome may even clear the commerce out of one of its marketplaces1 to some degree and devote the place wholly or partially to politics.
The Black Market probably doesn't operate from a specific marketplace - but on the other hand, its entirely possible for illegal goods to be sold "under the counter", or even openly, in a suitably poorly controlled market. Also, until very recently2, many places had a concept of "market ouvert" (French for "open market") whereby goods sold by daylight in a designated market passed freely to the buyer … in practice this could mean that even stolen goods could be "laundered" by selling them in such a market. Of course, this assumed that the thief would have to sell his gains locally and placed the emphasis on the person from whom they had been stolen to check the local market for his missing goods, but would also tend to mean that the thief could be easily identified (being either the seller or the person from whom the seller had bought the goods in question).
Another, quite popular, fantasy trope is the marketplace which exists in some outside place - typically a pocket dimension or boundary space. Depending on the narrative they belong to they may be places where straightforward goods are bought and sold for sensible prices in defined currencies, places where the highly exotic (and/or magical) changes hands for equally exotic goods in exchange, or truly bizarre places where price and value are determined on some baffling basis (such as some ill defined "significance" or the buyer's apparent desire). Such markets may or may not allow PCs to sell as well as barter … customers are typically welcome as long as they have money to spend (but getting to the market in the first place may be a challenge)3.
- Bazaar on Deeva - an example of the extradimensional market.
Game and Story Use
- Add flavour with the different marketplaces for different goods in your city - the noise and stink of the livestock market, a sweet and spicy marketplace catering for valuable imported materials and the bustle of the general goods … and, if applicable, the fish market (probably over and done with by the time the other markets are in full swing).
- Ethnic markets can also be very much a thing.
- Besides shopping you can also use a busy market place for all kinds of spycraft - especially brush-passes.
- As most cinematographers have noticed, busy markets are also great for chase scenes and dynamic battles - if the ordinary style of market doesn't suit, try moving it indoors to the crowded corridors of a middle eastern siouk … or something similar improvised in the winding alleys of an expy of Kowloon Walled City.
- Some markets breed a whole subculture - for London, for example, Smithfield has its meat porters, Billingsgate its fish porters and Covent Garden its costermongers…
- Searching for stolen goods in a "market ouvert" might be a fairly common job - expect the local thief takers to be first through the gate with a list of items that their clients want found…
- An annual market or "fair" can be a massive event in a pre-modern society - a make or break financial opportunity, a chance to acquire materials rarely available locally and generally an excuse to travel very long distances. Hosting such markets was a significant privilege, jealously guarded once acquired.