Unwarranted negative feedback

[coccinelle_2007]

I recently sold a pair of $20 Levi's jeans. I did everything according to eBay's standards... I sent the package on a Friday and it was delivered on Saturday. The tracking even said it arrived in the mailbox at its destination. The seller contacted me Saturday evening stating that they don't have the package. I said rarely does a package ever take one day to ship. They agreed that their mail usually comes late so they were going to wait a couple days to see if it arrives. I asked them to please let me know because this has never happened to me before. A couple of days went by and I checked my feedback... they left me negative feedback stating that they never received their package and that they contacted the seller and the seller said it was mailed. Never contacted me. Never opened a case or tried to get a refund. Does anyone else think this is super weird??? I contacted eBay right away and they said they can't do anything if they don't open a case. How is the seller protected against people who lie or leave negative feedback without trying to resolve the issue? They won't even give me a chance. What should I do? I tried contacting the buyer and they won't respond. eBay won't help me ever though tracking says the package was delivered and I did everything I was supposed to do.