So two things really - a natural market and a natural marketplace (group of similar markets).
Natural markets:
At the most basic level we have barter between trading parties. I have X, you want X; You have Y, I want Y. So we negotiate a rate of exchange for these items and this negotiation changes over time through circumstance.
The bees and the flowers do this sort of barter - nectar + pollen for pollen distribution. The ants and the fungi do as well - the fungus gives part of itself as food and the ants feed the fungus and distribute it to new locations.
IMO then, a natural market is a non-coerced market where there are just the parties trading involved - as opposed to what I think of as a coercive market, where there is a dominant third-party parasite involved - a party that doesn’t offer anything in exchange, but forces it’s way in and takes a percent of the trade for itself causing the market traders to alter their “prices” or ratios of traded goods/services in order to deal with the addition burden added by the parasite.
If the market parasites are overly successful, then they destroy one or more of the trading parties and the market dies.
Humans do the same, but have enhanced markets via money wich allows much more specialization of production - including tools, giving us more control over our daily existence. We invented money to make trade much easier, but such also has made it easier for the parasites to get involved in the trades and also for monopolists working with parasites – although monopolists do not survive for long in markets that lack coercion/parasites (see: Watch this video! :) - #482 by TylerAbeoJordan).
A natural marketplace:
Any individual market can fail at any time - Nature tells us that we can’t be sure of the future, so we living things, just pragmatically attempt to predict it and we trade based on our best guesses. The key for the survival globally then, is a diverse array of markets - separate traders each with their own pragmatic evaluations of reality. So that when nature throws a curve ball, not all markets will hopefully fail, some will thrive, others will hang on and adapt, others will be eliminated (the dinosaurs had one hell of a curve ball). This diverse marketplace is what I consider a natural marketplace. It’s also evolution - as genes (and memes) themselves are involved in a marketplace where they trade their services for their continued existence in the pool. Dawkin’s book “The Selfish Gene” introduced these ideas to genetics.
Balance for the connected planet is why diversity is critical IMO. Centralization vs decentralization is a constant battle. As things become hyper-centralized, the “economic calculation problem” makes the marketplace blind, accelerating it’s collapse. Just as any individual market always eventually fails a centralized marketplace can fail too.
So fighting to reduce centralizing forces (parasites and monopolists) moves us closer to what I like to think of as a natural “in balance” marketplace (group of markets). Whereas centralizing forces, parasites and monopolists fight to move things to an un-natural state by destroying markets - thus consolidating the marketplace and moving all marketplace participants closer to collapse.