If you are selling some stuff that is lying around in the attic then the answer is unlikely. To pay tax on what you sell you either need to be trading or make a capital gain.
HMRC are likely to consider you as trading where you are buying or making goods to sell them on to make a profit. This also applies where you sell these goods for a third party and you are paid a commission on your sales.
If you just sell odd items on Internet auctions, classified adverts or car boot sales it is unlikely HMRC will regard you as self-employed. This is because in most situations the amount you receive for goods sold is less than the amount you originally paid for the second hand goods you have put up for sale. Tax is only to be paid where you make a profit. You have have not made a profit here.
The same applies to capital gains tax. You only need to pay this tax on gains you made on selling certain assets. You will only make a gain where you sell the asset for more than what you paid for it or where the value of the asset has increased over the period your ownership and price you sell it for is above this increased value. The items you sell are more than likely to personal effects or goods – these are known as chattels, they are separately worth than £6k when you sell them. They are exempt from capital gains tax so it is unlikely that you will make a gain that is chargeable to tax.
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