They have the most genius game mechanic/ economic model.
So... You play the game to get loot. The more you play the more loot you get. So of course you wanna save the loot. So you put it in your stash( all players start with 3 stashes) , for safe keeping. The better it gets more loot you get. The moment you start getting the feel of the game (it is one deep game) you discover trading with other players. To trade with other players you discover premium stash tabs.... One tab is around 5 usd. But u need to buy game coins so u always have almost enough for one more stash.
So a premium tab is like a store front where u can display items for sale to other players. So naturally one starts buying more tabs. And more tabs. And then there are the specialty tabs that organize all the loot. Coz why do it manually when u can pay for a stash that auto sorts loot while you gather even more loot...
And suddenly you realize. You have 27 stashes for a total value of 300 usd... Most you have bought one the weekly stash sale.
And that is how PoE is financed. No walls no hidden stuff. Just loot.
yeh also cosmetics in case you wanna look good gathering all the loot.