Digital downloads from ebay

It's unlikely a company like ebay would sell bad merchandise and not have any customer support if something goes wrong with the product. I'd say go for it. If the key doesn't work, open a dispute. If they don't honor it later on down the line then reverse the charges from your bank and never do business with them again. That's been my attitude for any retailer but I've been lucky enough not to have had to do it yet.

I think as far as the grey market goes I wouldn't be using Ebay. God knows buying anything off there worries me enough as it is (in terms of is it the genuine product) never mind a game key.

Also if the key is retrieved via steam and later down the line it's revoked due to say, a stolen card aren't you given a limited number of strikes before you lose your account?