Let's be honest, it's not really going to do any damage to ebay. Ebay is too big. It's like comparing fiat to bitcoin. Sure they are different brands of the same thing but it's unlikely bitcoin is ever going to over take over or get close to fiat much like OB is to ebay.
It'll open up new avenues but its fate is very closely entwined with BTC itself. It'll occupy a healthy niche and unlike fiat there's a very powerful incentive to abandon Ebay.
All of Ebay's competitors have been weak imitations. This is fundamentally different in so many ways but when it comes to retail people are even more dumb and mindless than in other areas of their life.