For future reference, you should click on a person's profile, then click on their trust rating link. Then click on the "show untrusted feedback" link and see what's there. The user you paid has 10 negative feedbacks from the past several months where he's scammed other users for something.
What option did you use to send him the payment? Friends or Family? Or Good & Services? If you sent him payment for goods or services, you can dispute the transaction with PayPal and chargeback the payment. If you sent as Friends or Family, you're pretty much screwed.
Consider two things for future trades. First, only trade with trusted members (or use an escrow), and secondly, keep your trade discussions transparent. I personally won't trade with someone unless they've posted in the thread. If a user sends me a PM regarding a trade, I let everyone know publicly (in my thread) that the user reached out to me via PM.
I'm sorry you had to go through this, but it's your own fault for not doing your due diligence. Consider it a lesson learned (hopefully).
