How does poe.trade help you learn what does/doesn't sell whatsoever? The only thing that teaches you that is game knowledge about good/bad affixes. 80% of which you gain by playing and I'd say the other 20% you get by watching YouTube. Hell, I've been playing since slightly before breach and I still use poe trade macro for pricing shit. Because I don't have the time to fucking study every minute detail/affix on an item and how it rolled/interacts with the meta. It automates that process and improves the QoL of the shitty trade system so I can get back to the part that is actually fun and addicting - killing shit for more pulls of the slot machine.
An AH would actually make trading even riskier than the current system, thereby actually increasing the opportunity cost and forcing you to do more research before listing (especially if you're limited to X number of listings per day). How? Because in the current system if I list an item and get a torrent of messages in the first 20 seconds, I know I probably listed that bitch too low. Or, it's a low-cost item with relatively high demand (maps, leveling items, etc.). So, I can back out of the trade and relist the item for closer to its "actual" value. In an AH style system, that shit sells instantly and now I just cost myself a lot of currency.