Well, most the people that buy high-end IEM's from eBay are first time high-end IEM buyers, so they wouldn't notice the difference either way! Most 'audiophiles' buy IEM's from trusted sites supported by the IEM producer. This way you get a legit warranty and less problems
Personally I think it won't make a difference at all
If you pay via Paypal they insure your payment against fraud up to $200. They may have changed that. If you buy via Ebay you will also have to take warranty into consideration, some companies will only warranty sales from authorize dealers.
Bought my Audio techic CKM70's off ebay, from a company in Japan.
Ordered on the Friday, arrived on the Monday. Better packaged than anything i have ever bought before off ebay, and look/sound real enough to me.
If i am ever sceptical about a seller, i look at their history and feedback, and email buyers who have bought off the seller before (the same item), and ask them their honest opinions of the product and the seller. Most will reply, and be genuine about how they feel, and they can write a lot more information than what they can write on the small line on the feedback they have originally left.
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.