Hi Martin,
Our Share your items app doesn't include a Twitter button, but you can add your username to feature your last tweet in your listing description.
Also, once a buyer purchases form your store they are sent a post-purchase email asking them to share their purchase.
To read more about our Share your items & sign up visit eBay' app center: https://apps.ebay.com/selling?ViewEAppDetails&stab=1&appType=1&appId=ShareYourItems.ShareYourItems.com.
- Tzipi