I dibbled around in Photoshop some last night, and came up with a way in which an eBay listing with a Skype button might look.
Not will look, might look assuming eBay integrates its newest acquisition, Skype, into its search results listings page
What would be the upsell? An eBay lister could incorporate a Skype call prompt within their listing, indicating they invite possible bidders or purchasers to call them over Skype with questions.
When they click on your number, they start that Skype call..