If you win the claim they are blocked.
Why should sellers be able to arbitrarily block those who had a bad experience from leaving feedback? Getting a refund does not mean it was a good experience. They wanted the item as described, not a refund.
You can reply to the feedback left by the buyer, but cannot leave less-than-positive feedback for them, but you probably shouldn't. The ones you have left make you look like a hotheaded impatient seller I would not want to do business with.
Sellers can't leave negative (or neutral) feedback for buyers because too many sellers were using the threat of negative retaliation to discourage less than positive feedback from buyers no matter how warranted it was. And then if the threat wasn't enough, they would leave a scathing negative feedback for the buyer and seek a "mutual withdrawal." Since accurate and honest feedback for the seller is so much more important than for the buyer (all buyers have the ability to examine the seller's feedback profile before anything happens, while the seller is seldom able to do so and there are issues with taking action after the fact) eBay did away with negative and neutral feedback for buyers in 2008 and banned getting around that by leaving a negative comment on a positive rating.
On the other hand the Buyer Requirement that blocks those with 2 or more strikes in the past 12 months (or the more lenient settings available) acts automatically to block bids, offer, and BINs from serial deadbeats (IF their sellers used the proper unpaid item process) and so is far more effective than expecting to be able to react to feedback.
