As noted by others there are a couple issues going on here. First however you should know the problem/issue is not unique to eBay or any B&M store for that matter. What is referred to as "friendly fraud" has become a $100 billion dollar a year problem.
summary: buyer files bank chargeback against 16 year seller and gets to keep 7.7k in items for free without returning, eBays touted seller protection runs for the hills.
In the case of an INAD eBay claim the seller protection under the eBay policies comes into play and the seller can require the return of the items before issuing a refund to the buyer. However, if the seller does nothing in response to the buyers INAD return request the buyer can ask eBay to step in and eBay will fully refund the buyer and allow the buyer to keep the item. There is also no guarantee the seller will get back what they sent or if it will be in the condition they sent it in.
A INAD chargeback filed through the buyers CC company is a bit of different beast. First you should realize the CC companies hate chargebacks as much as the sellers or businesses. It requires resources to deal with those and that cuts into their bottom line. A INAD chargeback only looks at the financial aspects of the transaction and nothing else. The problem is the CC companies are forced to operate under a set of Federal laws that were passed 50+ years ago. As result CC companies have chargeback policies that you can access and review at your leisure I believe Master Card's is about 2,000 pages.
On the positive side CC companies do monitor cardholders chargeback history looking for abusive patterns and can revoke a cardholders account. They also monitor businesses for excessive chargeback claims and can revoke the businesses authority to permit the use of their card for purchases.
The only time a seller has blanket protection from a chargeback is for international sales that go through the eBay International Shipping (EIS) program.
I actually had something similar happen to me when a buyer went to their bank saying not as described, eBay let them keep the item, and made me give a full refund to them, and let them continue to use eBay like nothing happened.
That tends to be the general outcome. The same is generally true with eBay claims. While I don't do it often I have opened INAD chargebacks with my CC for items I have purchased either because the business or company did not have their own process in place or because the items were counterfeit. Since it is a felony to mail counterfeit items through the US postal service I used the chargeback process since I could not return the items.
