Go after them. Calm and reasonable at first but be prepared to hound them like a mad dog. Paypal's claims system does work but it'd designed to be hard to use so if you go that route, be prepared to jump through hoops. Most people will either cave in immediately or will wait til they see if you're serious, but with a bit of determination you should get something, either a full refund or a suitable part refund.
Oh, and obviously if you go for a refund don't send it back til you have cash in hand.
Of course, this might not seem worth the hassle. I tend to take it as a slight to my honour and so follow scammers to the end of the earth, never had one who didn't cave after I started small claims… But sometimes it's probably not worth it, I never know when to stop.