You must have a licence if you:
If you are in a partnership - every partner must hold a licence.
You must hold a certificate if you buy, sell or exchange for a licensed secondhand dealer, or if you issue pledge tickets for a licensed pawnbroker.
Schedule 1 of the Secondhand Dealers and Pawnbrokers Act 2004 list all of the items that are considered secondhand goods. If the items you sell are not on this list then you do not need to hold a licence.