I bought a key from ebay for a MW3 key on the 7th for only £21 which was a worldwide key and it activated fine.
However tonight Steam have revoked the key saying:

I have asked the seller to refund my money,but im not sure where i stand?
Will Paypal be on my side,as im not sure how i can prove the key has been removed from my Steam account.
And yes,i have learnt a lesson buying keys from Ebay in future
Edit:
The seller has sold around 15-20 keys for the same game
However tonight Steam have revoked the key saying:

I have asked the seller to refund my money,but im not sure where i stand?
Will Paypal be on my side,as im not sure how i can prove the key has been removed from my Steam account.
And yes,i have learnt a lesson buying keys from Ebay in future

Edit:
The seller has sold around 15-20 keys for the same game