20 emails in 2 weeks without waiting for a response? There's your problem right there.
Ebayers get their good rating when people's goods arrive on time and working, not for post sales customer service.
Warranties are now the equivalent of the old tail light warranty - your warranty lasts until the seller can no longer see the car's tail light, or in the modern world, until the PayPal dispute period ends.
Even the best sellers that come highly recommended here, that provide support well past warranty dates, have been known to ignore customers who send 3 emails on a weekend, then start send more asking why there's been no response before coming onto a forum, claiming "I sent hundreds of emails, but never got a response!" As if the harassment on their part, means the seller should have been more diligent, instead of doing the human thing and putting an auto delete on that customer. I think you'll find that very few people are going to avoid an eBay store with a good reputation, based on a forum post such as this. In some cases in the past, the vendor posted their side of the story, and gained a lot more customers, so it's backfired on the OP.
I can walk into a hobby store and pay $300 for a 5ah battery. Or I could order it online and get a 16ah one delivered for roughly the same cost. With the store, I expect to be able to bring it back if it fails. On the internet, I expect it to have no warranty regardless of what the seller says, because the cost of shipping would be half the cost of the product anyway.
As the saying goes, "Buy Cheap, Buy Twice". Or sometimes three or more times.
All the best to you. Your best bet now is to ask for advice regarding how to fix it here, instead of slagging off a vendor.
