What would a good trading system look like?

PoE's trading system by most accounts is a bit out-dated. I personally don't find it too hard to work with - and in some ways enjoy geeking out with it - but still, it's not at the level expected from a modern ARPG or MMO.

I'm not the most experienced ARPG player, so someone would need to enlighten me about how it works in e.g. D4. I've some experience on MMOs and looter-shooters though.

In games I've played, there's definitely been a bunch of negative sides to trading houses. In GW2 and ESO for example, buying the stuff you need is by far the most time effective way of getting things you need; it's pretty much only the stuff that can not be traded that is better farmed yourself.

In Warframe, which also doesn't have a trading house, trading utilizes the in-game money primarily bought for with real money; you can get the in-game money via e.g. trading yourself, but still, the fact is that the system significantly shortens the grind for players using real world money to get gear. The end-game is so much about fashion and trading is a bit less necessary and less used that there lack of a trading house isn't a big problem.

All those have a trade tax. The purpose of the tax is not really to slow trade down, but to work as a currency sink that helps combat inflation, and add a small disincentive for price-gouging.

So far the best system I've seen is in ESO. In that game, you have to go to guild trader NPCs to buy stuff, and those are physically spread around the world. So it takes a little bit of effort to buy stuff. This is good, because it adds a little bit of incentive to also farm stuff yourself if you think that it's relatively easy to farm the stuff. Also, as the trader NPCs in the most popular locations are limited and they need to be bought as a guild, there's some incentive for guilds to compete with each other and this creates a little bit healthier economy.

In all these systems, the trading becomes by far the best means of gaining all your gear, as long as it is tradeable. In some ways that's a bit sad, because it makes you more concerned about farming currency than farming any gear itself. That has of course also happened in PoE1 and 2. Still, I do feel that adding effort to the trade system like in PoE is a meaningful additional tax to doing trade. An imperfect one, but meaningful anyway.

In all honesty, I don't know what a trading system should look like. I think a naive trading house implementation would be no better than what we now have, all things considered.

But I'm sure something better could exist.