cash from ebay and tax credits!

Hi all,
I am a stay at home mum with a toddler and my husband works full time but is on a low wage so we get tax credits. My question is... I am going to sell some of our excess belongings and kiddie clothes on e-bay should I declare any money I make to the IR? I'm not planning on earning a fourtune but I dont want to get in trouble!
Can anyone help please?
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