I just got through a dispute/claim for a ratchet I paid for and didn't receive. My only advice is to be honest and respectful in your communications to the buyer through the resolution center. This does 2 things. It gives the seller no reason to get defensive but in the event that they do, paypal will look at the communication and see that you've done everything right and honestly and will likely find in your favor. If you start accusing things and getting your back up, you'll reduce your chances. Nobody likes a jerk! Also, make sure and give the seller options for resolution. I gave 3 - refund, send another of the same item or send a replacement item of equal value of my choice(provided he had it). if the resolution doesn't work and you come to the point when you file the paypal claim, state your case thoroughly and again, honestly to paypal. My claim was settled in less than a week after I filed it.
**edit** Good Luck!!!