Most of these brokers cover their losses by different kind of fees, and they do it well it seems. Paypal is one of them and the worst fee is if you get paid in one currency and then either want to pay a supplier in another currency or want to transfer the money to your bank account that is in another currency. No one has as bad exchange rate as Paypal PLUS a fee. The big credit card brokers mainly use the official bank rate
Our Swedish broker uses a system where they make a credit check on the buyer for every purchase. It is done without human intervention during the checkout
Fraudulent buyers and sellers have always existed and will always do. It has become easier now with e-shops and credit cards. But the card providers also make it easier. My new bank card uses wireless technique to just be placed close to the machine and your payment is done without even using a pin. Even if the amount is limited It is a dangerous way.