You're probably confusing with how the Steam Market works.
There is a "Steam Fee" which is 5%, but there are also "game specific fees" for Dota2/CSGO/TF2 (each at 10%).
When you sell an item on the market, you're asked for two values: "Buyer pays" and "You receive", which are relative to each other. (As in: Changing one box will change the other).
This means that every time you sell an item on the market, Valve and the creators of the game (in this case; Grinding Gear Games, for Path of Exile) will get a cut of the profits you received when selling the item.
For example, if I was to sell a Path of Exile mask for $10.00, I would receive $8.50 (my cut of 85%), Valve would receive $0.50 (5% cut) while the creator of the game; Grinding Gear Games will receive $1.00 (10% cut), while the buyer has to pay all $10.00 of it.
CS:GO/TF2/DoTA 2/Portal/Steam items would result in Valve receiving a 15% cut (5% from standard tax and 10% from owning the game) etc.
Also, what verification documents does Steam require for big sellers?
When you hit 200 market transactions (or $20,000 USD worth), whichever happens first, you'll be blocked from using the market further until you fill up a tax information form. For non-US customers, this shouldn't be a problem, as you will not be charged extra tax.
For those who live in the US however (and exceeding the $20,000 USD threshold), income will be reported to the IRS and taxes will have to be paid as income tax, outside of the Steam Marketplace. You'll get a 1099-K.