Hi Abbie9
What is your tax status with HMRC? If you are self employed with them then so also with the DWP and again if you are unemployed as per HMRC then the same with DWP.
According to UC rules you don't have to inform them if you sell your personal possessions but if you buy stuff to sell on whether online or at a car boot sale then you do.
David
Nothing on this board constitutes legal advice - always consult a professional about specific problems