If you are just selling some unwanted items that have been lying around in the attic and your home, the answer is probably no, as in order to pay tax on the goods you sell, you either have to be trading or make a capital gain.
You are likely to be treated as trading where the HMRC consider you to be purchasing or making goods for resale with the intention of making a profit, or sell goods for others and receive a commission.
If you only sell occasional, unwanted personal items through internet auctions, car boot sales and classified advertisements then it is unlikely they will view you as self-employed. This is due to the fact that in most cases the second hand value and amount you receive rarely exceeds the original price you paid for item, and as tax is only chargeable on the profits you make, no tax will be chargeable.
Capital Gains Tax is only charged on gains you make and if you sell an item for less than you purchased it for you will not make a gain. It is the gain that is taxed, not the amount you receive. Therefore, you will only have to pay tax if the items you sell have increased in value during the time you have owned them, and as they are likely to be personal effects or goods (known as ‘chattels’) you are selling, which are individually worth less than £6,000 when you dispose of them, it is very unlikely you will have made a gain.
If you are concerned that your situation may be considered to be trading you should discuss this with your local TaxAssist accountant, as there is a late registration penalty if you do not register within 3 months of starting to trade.