The only way to inform HMRC of anything officially is via a self assessment tax return. The deadline for online tax returns has now passed.
Why are you asking?
If you are only selling your own pre-owned items, you don't have to pay tax.
If Ebay has passed your details for last year to HMRC, they will contact you to ask for more information if they feel you have a tax liability.
"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)
