It depends. When in doubt, I'll offer up the disclaimer that if they need warranty support I'll provide a copy of the original purchase receipt within 'x' days of auction close (just so I don't have to keep around forever). I'm not providing new stuff, so, typically it's a 90day thing in my auctions.
With the receipt the warranty should be valid, because who are they to say if it was a gift from a friend or something.
As a buyer, I basically don't count on warranty via ebay, unless from a legit company as said above or products where a receipt isn't needed.