Over the years trading on GB the most common form of payment is USPS money order. Federal crime if things go wrong. Buyer sends MO, seller ships. If the seller defaults the feds WILL get involved.
I know people say get a USPS MO because its a federal crime etc but have you ever seen anyone use a USPS MO, get shafted, the feds get involved and they get their money back or their item? Because I haven't. At the end of the day you're still out of your hard earned money.