well taking the troll bait here, and replying.

First it creates a hard limit on how much people can sell, and how quickly someone can alter prices on the market. Sure there are ways around this, ie more retainers, and multiple accounts, but in the end SE makes money when people try and bypass the limitations of the system.

I'll use WOW's AH as an Example. you can post as many Auctions as you want, you can use addons to scan what is on the Auction, and set prices to manipulate the market quickly and efficiently. Gold Sellers make a huge profit grinding out crafting materials and posting them on the AH at rates lower than most players feel is worth the grind. One player can have thousands of Auctions of the current raid their crafting materials, making profit grinding less effcitiant for normal players.

Here, you have 40 slots on the market with your paid account. Hard limit. You can pay money for 20 more slots aka an additional retainer. This reduces the ability of goldsellers being able to flood the markets. They can and do, but it costs them a lot of money to do so. As they have to have a paid account, with additional retainers, and usually additional characters all in the same FC so they can transfer inventory, and have more than the base 40 slots. This takes time, and energy away from the collection grind, forcing the gold sellers to have to have additional accounts just to farm the crafting reagents.

So unlike WOW which one character can do all the work, grind and sell, here the gold farmers have to have multiple accounts, multiple retainers, multiple characters, and they can't keep track of the market instantly like they can in wow. Basically they have slowed goldsellers rate of growth significantly by using the retainers. Killing the market for them, and making it easier for SE to track sellers here.