I believe (I could be mistaken) they act as the middle man and they hold the funds during the transaction. If the buyer encounters any issues with the machine, he has to return it to you before they give him his money back. If you want a full explanation of how it works, check out Vid's guide below post # 7
https://pinside.com/pinball/forum/topic/how-to-not-get-ripped-off-in-pinball-vids-guide