Seller Silence

Warranties typically apply to the original purchaser and aren't transferable. That may be why the sellers didn't respond to you.

There are some notable exceptions, such as Bryston.

I understand your frustration. If you get interested in buying something and are met with no response. It has happened to me and by and large has resulted in me not shopping used. When I make a decision to pursue something I want to move forward. A pre-sales rating is an interesting idea. 

There is absolutely no obligation for a seller to answer your questions,  though it certainly would be courteous and beneficial to both parties. 

I see courtesy as a kind of obligation, one that especially benefits the seller. That said, the buy has the option to walk away or, if the purchase happens eventually, leave a rating of the seller that downgrades them for the communication component of the transaction.