Sold date is 28th Oct. Feedback left on 3rd Nov.

Even if you posted on Monday 29th Oct the buyer didn't receive the seed till 31st. The buyer was saying "it didn't grow" within 3 days she had the seeds.

Ring up eBay and tell them about how short time the buyer had the seed and the feedback is not valid.

Your feedback to the seller is valid as you had the seed for a month. If so why didn't you take it?

I'm not a gardener but I have planted many seeds in past. Not all seeds grow. You get bad ones sometimes. Your negative feedback is a bit harsh if the seller offered replacement or refund.