By acting as the intermediary they are the only party that received the posters money and ergo the only and appropriate party to claw the money back from. This is no different from buying a widget on amazon. Failing to receive the widget. Initiating a chargeback for the money.

Regardless of whether amazon was actually responsible for providing the widget they received your money and ergo they are responsible for giving it back.

Indeed its not really any different from a direct retail purchase where you buy a box and if you open the box and instead of nice new widget you find one that looks like it was mauled by a bear. Can you imagine going to the returns counter and being told despite paying us your money at our point of sale you need to fly to china and sue them if you want your money back. It's just not how the universe works.

They most likely actually CAN'T take money and deliver nothing and make it someone else's problem if they both want to do business in the US and keep doing business with credit card issuers here.