I have a first generation box donated by my boyfriend's mum, and a Sky Yellow House card purchased off eBay some time ago. They work together fine.
However, I've recently tried to list a Sky card on eBay (when an elderly relative died we were given their box and card too), and Sky have removed the listing because of some arbitrary "intellectual property" rubbish. To top it, the email address eBay give you to ask Sky what the deal is bounces.
Personally, I think Sky are putting around a certain amount of misinformation on purpose. The cards do work, so just ask the seller if you can return it if it doesn't work and if they say yes, nothing lost.