If you are a private seller, selling off your own no longer wanted bits and bobs, you don't need to declare anything to HMRC.
If you are thinking that you might have the makings of a business and are trying it out, you can sell £1000 turnover without declaring.
If you are a business seller, buying, making or growing to sell, then you need to declare everything over that £1000 - but you can claim expenses on the amount over the £1k.