Replying to Ebay and Manufacturer's Warrenties

I searched ebay and warrenty and couldn't seem to come up with anything on this subject. But if there are old threads on this topic, let me know.

But my question was if anyone knows how warrenties with skis/bindings etc. work, with stuff bought on ebay? Are the warrenties void because they were not sold retail? And like as far as a 1 or 2 year warrenty is concerned, how can they be sure when the date of purchase was. Like if you dont get an official store receipt.

Do packing slips and invoices, or ebay printouts work or paypal histories work?