You have sold over 30 items so Ebay have to report your details to HMRC. You need to supply them with your NI number.
You do not usually have to pay tax if you are selling your own unwanted items. If you don't work, you still have a personal allowance of £12750 (think that is right but have not checked) so you are unlikely to have a tax liability for selling on Ebay.
There are several threads on this topic on the discussion boards so it may be worth reading through some of them.
Honestly though, the best place for tax advice is HMRC.
"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)