Let’s first realize that for the most part eBay sellers don’t want to talk with their customers, except maybe for high ticket items like boats and cars.
Second, one has to question why eBay is only allowing Skype buttons and not others from Jajah, GrandCentral, SightSpeed, etc. all of whom may already be in place by the buyer or seller. I mean, eBay sellers accept forms of payment other than PayPal to get paid and always have.
What would make more sense would be to give sellers free Skype In numbers with Skype Out optional forwarding and voice mail, then bridge the call to that type of service using ANY click to call service, through an API. Skype/eBay would still get what they want, and like with PayPal, they would pick up users who use Skype to Skype calling (this would be another angle for the 3 Skype phone button.)
At the end of the day, it’s the eBay transaction that eBay wants, not how the call was made to enable the conversation, so anything that fosters