If for example someone plays a blacksmith and needs coal/iron and sells swords, but the miners searching/finding iron/coal dont need swords but mules for transportation, but people selling mules need to feed those and farmers selling food need fertilizer and people selling fertilizer need bags and people making bags need leather and people finding leather from monsters need a sword? And there can easily be many other such cyles, which could easily be fulfilled by everyone trading for money, but get near impossible to solve with only bartering between 2 people allowed, when everyone would shy away from accepting something they dont immediately need and they dont have experience in how good it would resell and no connections to other people needing it.

This is a good thing to note. However, I wonder if the trading system could make these connections? Automation is something I'd really like to be heavy on so the trading system becomes easier. If I'm a blacksmith I put up for trade swords, and I say I need coal, and the system could cycle through all offers/needs and make the connection n deep (say 5 deep connections)? So in your scenario it could automatically make those connections and inform all parties where each party has a chance to counter offer or just accept? Maybe when you put your offer/need up you can give a range of quantity of the item you want and if everything is within this range the trade is just done. This could help reduce counter offers slowing things down.

Just imagine you put up 1 sword and want between 2-5 iron AND 2-5 coal and it just happens automatically but behind the scenes it connected 5 other trades! How cool would that be

All of this information of every bid/offer could be recorded and shown to the players to give them an idea (avg) of what is going for what. You could even get instant feedback if there are enough players in the game. You could say you want coal/iron and it would show you the avg cost of what you currently have in your inventory or if you don't have anything it goes for it could list out what it would go for from things previously in your inventory making the assumption that you are a certain type of player. ie. if you are a farmer you'll mostly have food in your inventory and the system would know this etc. Could get really fancy with the automation.

The thing to remember is that this is a video game and therefore has better ways to automate things than the real world.

If you want to make it that complex, why automate it ?

I think there are plenty of players who would enjoy trying to make these X-deep trade-cycles, other players would only have to put up what they have/want and accept a trade-cycle proposed by those players.

I just don't think players would want to deal with that, or at least not enough players to run the entire system. I wouldn't want the game itself to be about this. This is a means to get other players what they want for the cost of things they have. I also see this as a perfect task for automation. Even in the real world. I want the players of the world to be dependent on each other. If you can't do everything, mine, blacksmith, farm, miner, etc, then it makes players focus on 1-2 tasks and depend on other players for their other needs. I'm more interested in the dependency instead of direct interaction.

Enough players would like to deal with the system if it works, if it is easy to understand and if it is not too time-consuming.

The only way of getting rid of gold-farmers is to make trading so annoying/complex that people rather get what they need instead of trading for it.

In this regard i suggest you make getting "what you need" fun/easy/accessible/profitable/... to the player.

I think a dedicated gold farmer is much more tolerant to a frustrating/slow collecting/trading process than the average player. That would backfire, if more frustrated players get to be tempted to grow into customers of botters/farmers/goldsellers and others stop playing cause of their frustration and seeing others cheating.