Trading in Path of Exile was done mainly through poe.trade and its currency equivalent currency.poe.trade. The problem with these platforms nowadays is that GGG released their own trading platform, which has tons more filters and is way more responsive and basically takes no time to update. It is also more stable and takes less time to load pages, resulting in a way smoother feel while trading.
As GGG said we will not be getting any kind of Action House so we will simply have to embrace the trading we have right now and honestly, taking your time to acquire your items that way is kind of fun instead of picking them off your mail box against a certain fixed price.
There are a lot of extensive guides that teach you how to use the Path of Exile Trade Search. We recommend checking out this small Youtuber that does the job of explaining very well.