ebay doesn’t want sellers to communicate in any way other than through their messaging system. It’s the same way on amazon. Like many other rules and procedures, it came about because sellers were abusing the privilege of free communication (for example, a potential buyer would ask a question of a seller and the seller would tell the prospect he’d sell it to him directly).
In any case, if the seller is willing, he can photograph pages of the manual you’re interested in and attach those to the ebay emails.
I have had photos of an email address and web page sent through eBay messaging. Even included my website in pictures of my products for years.