Seller Silence

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.

Cleeds has it right.  If the buyer wants this info, the seller cannot provide it, only the manufacturer or importer can. 

There can be a number of reasons the seller didn’t respond.  Send them a second request and if they don’t answer, move on.  Give the seller a few days to respond, not weeks.

If a seller doesn't see fit to respond to potential buyers questions, for whatever reason, although that's the sellers prerogative, obviously the seller must not need/want to sell the item bad enough.  No sweat.  If the negative responses bothered me that much, I'd quickly forget about that seller and look elsewhere.  Happy listening