How does "Velocity" know you have registered?
It works on affiliate links using special identifiers. Basically when you log onto the velocity page, they know who you are. When you click on a shop, you (and a random identifier that velocity knows) are passed to an affiliate type company. This then connects you to the shop.
When you make a purchase, the affiliate earns commission (and has a system of being able to identify the identifier used using cookies /entry time into shop / a unique code) . This then gets pushed back to velocity as the 'pending' status. Velocity knows who you are from the random identifier.
The shop themselves normally have to 'approve' the payment of the commission (in case you return the goods).
This works well normally, but sometimes has issues. In the ebay case, I assume they will post after 30-60days by themselves.
Plenty of similar websites in Australia and worldwide that use similar technology. An Australian exampe is Buckscoop - Your cashback and shopping community , an international one AAdvantage eShopping(SM) program or Cookies are Required