i've had plenty of problems from "sellers" on ebay, the worst is the Meizu MP3 player that I never received. the main thing is to get in quick if you think things are going south.
don't hang on a seller's promise. give 'em one chance to sort things out then hit them hard. contact paypal within...45 days (now) and open up a dispute.
if the seller tries to slow the process down, don't listen. Like i've said, i've been done big time, and have trusted the seller to come through. in the end i've always come off worse. now i get in there quick. stuff their seller rating, it's your heard earned we're talking about.
if you have any resorvations, remember that people also sell used gear on amazon. I've had no problem from amazon sellers i have to admit, and i've bough loads of used headphones from the amazon site.
please remember the only real difference between amazon anbay is that ebnay you bid. having never had a problem from an amazon seller, i can't say how good the customer service is, because i've never had to use it.
there are some genuine stores (or that's what oi thought when i bought my Meizu) on ebay and some good bargains.