Ads are displayed on ebay listing using Adchoice, and you can opt out of these ads.

http://goanonymous.org/2011/06/waste-of-money-in-ppc-cpm-advertising/

Even after opting out of the customised ads in Adchoice, I was still shown the GIA jewelry ads. On clicking the ad, it appears that Doubleclick , owned by Google is showing the jewelry ad for GIA. A case of extremely poor targetting and waste of advertiser money and since the event has already taken place a few months ago.