They will be asking for yours at some time in the future.
If you refuse to provide it, they will still forward your other details to HMRC, probably adding that you refused to provide your NI number.
Although you have not bothered to register as a business seller with Ebay, and your 'no returns' policy breaks consumer law, i assume you are already completing a tax return and paying tax on your sales.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
