This only shows you've never really thought about it in-depth. D2's system is MASSIVELY broken. 99.99% of (non-trash-drop) items are straight-up worthless. Even end-game worthy pieces that aren't top-tier rarity (like, say, Trang-Oul Gloves) are absolute vendor trash 1 week into the season. Nothing has value except for high runes, and a small handful of uniques - and of course god-tier rolls on magic/rare items. Take set items, for example. You know how many set items have actual value? By which I mean more than a casual few PAms or a Lem rune, which is just one step above vendor trash. The answer is TWO. Two set items. In the entire game. And for uniques, too, the pool of actual worthwhile drops is absolutely minuscule. Barring absolutely exceptional rolls, there's like 3 or 4 uniques that are even worth a high rune; all of which are TC87, meaning they're so rare you can easily go an entire season without ever finding one. And some of the god-tier magic/rare items are even rarer than that, to the point where even streamers that farm ungodly hours for an entire season will not find some of the regulars (like e.g. JMOD), let alone the top-tier rares that are akin to a lottery win. And that's not even talking about rune words, which have created a toxic meta all on their own. Items like Enigma, Grief, and Infinity devalue entire catalogs of strategies, and are so far above anything else in power they make non-rune drops look like a laughable waste. Even a low-level runeword like Spirit effectively kills every low to mid-level drop item in its slots, making every class's season start nigh-on identical because it's stupidly powerful and stupidly easy to get.
Anyway...
But you are very confidently asserting that there are "tons of ways"?
This was already discussed to death years ago in D3, and the result is: it doesn't work. All this does is incentivize degenerate gameplay where farmers sell guild slots to people so they can get fed gear. This is not something anyone wants or needs.
The logistics of such a system are a minor problem. The actual issue is what you yourself already mentioned initially: the game becomes about currency farming and maximizing trade value, which means less time beheading demons and more time scouring AH deals at 5am to snipe some listing and flip it for more profit than you could ever realistically farm. Also, a centralized currency system MASSIVELY devalues drops simply by virtue of its existence, since the easier it is to exchange things the more supply and demand push the market towards the exceptional, ultra rare drops. Which means the regular drops normal people can find very quickly become very worthless, as there's just too much supply for anything that isn't insanely rare - which also means the average player will never find it, but has to buy it, FURTHER exacerbating the trade problem as they're incentivized more and more to worry about currency instead of item drops.
That's one option, and it was mentioned in the D4 context some time ago; I'm unsure where they currently are on trading. This seems like an "eh, whatever" solution, as it largely concerns items of little to no relevance, which also means they'll never be worth a lot and people can easily just skip the entire thing. Which means... why is it there, exactly? Why add a point of failure and frustration with scammers, and/or additional logistics for a system people will maybe use here and there while leveling up but never bother with again once they reach endgame. Seems like a waste of development resources.
You'd need to demonstrate it provides long-term value, first. All well and good to claim it - prove it. If you want to go with D2's longevity (which is largely a niche audience, by the way), prove it's the trading and not something else that's behind that. As for the "community argument"... anything can build "communities", that's something that happens almost on its own. You'd have to show that trading does this either better than anything else, or in ways that are different than others - and then, of course, also show that these "communities" ADD value rather than subtract it, as the one concrete example you've mentioned (JSP) has a highly controversial, if not downright toxic effect on the game. If anything, it's an argument AGAINST trading, not for it.