Traditional markets in open air were set up in public places.
People could easily visit all vendors present at a place but it was not well ordered, so that it was difficult for consumers to find their way there. It was a waste of time and a matter of chance.
Waste of time for consumers searching for what they look for, was a sort supplementary cost.
Then came supermarkets, where the diversity of objects on a market were all available in a more structured way so that people could more easily find all what they were looking for.
The problem is that a supermarket is itself a private place.
Consumers save time but they pay it back in the form of profit margins for the supermarket itself.
At the time of internet, a similar problem arises.
There is a galaxy of independent vendors that is not well ordered, and where finding what we want is hard, hasardous and time consuming.
And there are supermarkets like eBay that take a profit margin on the transactions of the whole world.
The problem is that it would be hard for a market to be both perfect and in perfect competition with other markets on the same market.
Consumers need to have a sort of unique address when searching for anything, because they can't spend their time trying many ways to search for and compile by hand the lists of offers for the same object. And the problem is how this unique address can be the one of a public space.
There needs to be open standards for objects on sales or other offers, and open systems to systematically collect and compare the data expressed according to these standards.
There are different types of markets that would need to be treated in different ways:
For each object type are many copies, many buyers and many sellers. The price is determined by the classical law of offer and demand.
For each object type are many copies, many buyers but only one seller (Typically, it is a trademarked object). Usually, the price and type of the object are arbitrarily fixed by the seller.
For each type of object, only one copy (only one occurence of the interaction is possible), whose type is determined by only one of both parties. This one only considers the price, so an auction occurs between the many potential buyers.
The type of the object (or rather of the considered relation instead of a material object) depends both on buyer and seller. From each side, no two potential partners would be the same with a mere price difference. Reviewing work has to be done by both sides. See details for Open Matchmaking.