USD is the world currency and that's why it is used. It would be tedious and expensive to have markets in many different currencies.
As for transferring money around, paypal is one of the best options when using just one currency. I personally have used previous and still use a service called transferwise to save money on fees when converting funds from one currency to another.
Well if the transactions happen in the same currency, it will be much better than actually making it in multiple currencies which further involve the conversion of one currency to another and so on which makes chaos.