I like to use paypal, it's free and a lot more convenient than going to the post office. It's quicker than mailing a check or money order as well.
The only downside is that once you send the payment with paypal, the person you're dealing with had better be honest, you can't stop the payment or get a refund if the seller is a scammer.
I only deal with people I know or who have good references for that reason.
Trips