3rd party sellers

I don’t understand how so many YouTubers suggest to buy cash, pool coins, pool pass etc from them at a cheaper rate. I mean who are they to sell us cash and how will they do it? I can understand they giving us pool coins by inter-account trading or by using user account to earn them. But how do they claim to be sellers of cash and pool pass.

Can someone explain what goes behind this, do they actually hack the resources for you or what?